Porous media, and especially phenomena of transport in such materials, are an impor1ant field of interest for geologists, hydrogeologists, researchers in soil and fluid mechanics, petroleum and chemical engineers, physicists and scientists in many other disciplines. The development of better numerical simulation techniques in combination with the enormous expansion of computer tools, have enabled numerical simulation of transport phenomena (mass of phases and components, energy etc. ) in porous domains of interest. Before any practical application of the results of such simulations can be used, it is essential that the simulation models have been proven to be valid. In order to establish the greatest possible coherence between the models and the physical reality, frequent interaction between numericians, mathematicians and the previously quoted researchers, is necessary. Once this coherence is established, the numerical simulations could be used to predict various phenomena such as water management, propagation of pollutants etc. These simulations could be, in many cases, the only financially acceptable tool to carry out an investigation. Current studies within various fields of applications include not only physical comprehension aspects of flow and energy or solute transport in saturated or unsaturated media but also numerical aspects in deriving strong complex equations. Among the various fields of applications generally two types of problems can be observed. Those associated with the pollution of the environment and those linked to water management. The former are essentially a problem in industrialized countries, the latter are a major source of concern in North-Africa.

Transport phenomenain porous media are encounteredin various disciplines, e. g. , civil engineering, chemical engineering, reservoir engineering, agricul tural engineering and soil science. In these disciplines, problems are en countered in which various extensive quantities, e. g. , mass and heat, are transported through a porous material domain. Often, the void space of the porous material contains two or three fluid phases, and the various ex tensive quantities are transported simultaneously through the multiphase system. In all these disciplines, decisions related to a system's development and its operation have to be made. To do so a tool is needed that will pro vide a forecast of the system's response to the implementation of proposed decisions. This response is expressed in the form of spatial and temporal distributions of the state variables that describe the system's behavior. Ex amples of such state variables are pressure, stress, strain, density, velocity, solute concentration, temperature, etc. , for each phase in the system, The tool that enables the required predictions is the model. A model may be defined as a simplified version of the real porous medium system and the transport phenomena that occur in it. Because the model is a sim plified version of the real system, no unique model exists for a given porous medium system. Different sets of simplifying assumptions, each suitable for a particular task, will result in different models.

On February 19, 1973, five centuries have elapsed since the birth of Nicolaus Coperni cus - the greatest astronomer of the Renaissance period - who rediscovered for us the heliocentric model of the solar system, and documented it by his life's work in such a manner as to make its concept a permanent property of mankind. The life of Copernicus, extending from 19 February 1473 to his death on 24 May 1543, was not too rich in adventures or biographical facts. Born in Toruti from a family of Polish burghers, he received his first university training in Cracow between 1491-1494. From Cracow he proceeded to Italy to spend the years between 1496-1503 at the Universities of Bologna, Padua and Ferrara - with occasional visits to Rom- in preparation for an ecclesiastical career. When Bishop Watzenrode - his patron and maternal uncle - could no longer extend his leave, Copernicus returned to Poland in 1503 to enter the service of the church establishment, which soon led to a canonry at the Frombork (Frauenburg) Cathedral in Warmia. And there - in the northern mists not far from the Baltic shores - in a land so different in climate from the sunny Italy of his youth - he was destined to spend the rest of his life."

Gian Gaspare Zuffa Dipartimento di Scienze della Terra Universita della Calabria 87030 Castiglione Cosentino Stazione, Cosenza, ITALY The tradition has been to consider Sedimentology and Sedimen- tary Petrology as quite separate areas of research. It is however impossible to arrive at an optimal description of sedimentary rocks without integrating the two fields since sedimentary processes and compositional aspects are strongly intertwined. The study of arenites is of particular importance in obtaining paleogeographic and paleotectonic reconstructions aimed at deter- mining the geodynamics of the earth's crust. It also has important implications in exploration for and exploitation of hydrocarbons. At the NATO ASI Meeting on Reading Provenance from Arenites held in Calabria (Italy), June 3-ll, 1984, field sedimentologists and sedimentary petrologists were given opportunity to pool their resources in order to obtain better analyses of both source areas and depositional basins. The papers collected in this volume represent an edited ver- sion of the lectures given and provide a comprehensive picture of the present state of the art since they include such important top- ics as: l) the climate and relief of the source areas, 2) mechani- cal transport of sediments and depositional processes, 3) postdep- ositional processes, and 4) the methodology adopted for petrogra- phic optical analyses. Particular attention has been paid to the limitations and errors introduced into paleogeographic and paleo- tectonic reconstructions by incomplete and incorrect data.

More than half of the world's petroleum is to be found in carbonate rocks, for example in the Middle East, the former USSR and in North America. These rocks show a bewildering diversity of grains and textures, due in part to the wealth of different fossil organisms which have contributed to carbonate sedimentation, and in part to a wide variety of diagenetic processes which can radically modify textures and obscure the depositional fabric. Careful petrographic study with a polarising microscope is a key element of any study of carbonate sediments, as a companion to field or core logging, and as a necessary precursor to geochemical analysis. This atlas, which illustrates in full colour a range of features not attempted in any general textbook, is designed as a laboratory manual to keep beside the microscope, as an aid to identifying grain types and textures in carbonates. It appeals alike to undergraduate and graduate students and to professionals in teaching institutions, research laboratories and industry.

The interpretation of ancient sedimentary environments is a vital tool in the search for petroleum, coal and other economic resources. This fourth edition of this text has been rewritten to reflect advances in the subject, in particular stratigraphy and new geophysical techniques. New case studies have been added where appropriate.

In the late 1990s there has been a dramatic increase in the number of mathematical models developed to tackle geomechanics problems, largely as a result of the increasing availability and power of computers and the parallel evolution of versatile numerical techniques. This text examines the experimental and theoretical aspects of the pre-failure and post-failure behaviour of geomaterials within the frame of bifurcation theory. Coverage includes basic continuum mechanics for dry and fluid infiltrated porous media, bifurcation and stability analyses applied to layered geological media and granular materials, and theories for generalized continua (Cosserat and gradient theories) as applied to materials with microstructure and in relation to strain localization phenomena. This volume will be a source of reference for researchers and practitioners in soil and rock mechanics, and foundation engineering, and engineers, geologists and structural, civil and geotechincal engineers.

model. They conclude that the models using three fitting parameters provide the best fit over a wide range of suctions. Models for soil-water characteristic curves are only useful if we have experimental data on which to base them. Agus, Leong and Rahardjo (Singapore) present a large number of experimental soil-water characteristic curves determined for two types of residual soil from Sigapore. They present data for eight different sites. This data set allows them to relate the parameters of the soil-water characteristic curves to index properties. They conclude that the relationships derived are suitable to pro vide a quick preliminary estimate of a soil-water characteristic curve. The importance of soil-water characteristic curves is emphasized by another con tribution dealing with this topic. Aung, Rahardjo, Leong and Toll (Singapore) inves tigate the relationship between mercury intrusion porosimetry measurements and soil-water characteristic curves. The porosimetry measurements are presented as soil-air characteristic curves. The slopes of the soil-air characteristic curves are found to be similar to the slopes of the soil-water characteristic curves. The equiv alent pore diameters calculated from the mercury entry value and the air entry value appear to be related. Therefore, it is suggested that porosimetry data can be used to construct an estimate of the soil-water characteristic curve."

""" This book has been written as a guide to the management and use of formulated feeds in intensive fish and shrimp culture. While its focus is on the use of commercially pro duced feeds in intensive production systems, it is anticipated that many of the practical issues covered will be of equal interest to those fish farmers who make their own feeds and to those who use formulated feeds in less intensive systems. Feeds and feeding are the major variable operating costs in intensive aquaculture and the book is primarily in tended to aid decision making by fish farm managers in areas of feeding policy. The dramatic increases in aquaculture production seen over the past 15 years have been made possible, in large part, by gains in our understanding of the food and feed ing requirements of key fish and shrimp species. A global aquaculture feeds industry has developed and a wide range of specialist feeds is now sold. The new options in feeds and feeding systems, which are becoming available, necessitate continual review by farmers of their feeding policies, where choices must be made as to appropriate feed types and feeding methods. While growth rates and feed conversion values are the prime factors of interest to farmers, other important issues, such as product qualiry and environmental impacts of farm effluents, are also directly related to feed management practices."

Partial melting occurs in a variety of geological environments, from granitic partial melts in the continental crust, to basaltic or carbonate partial melts in the upper mantle. Partial melting is the first stage of magmatism and therefore plays a role of primary importance in the chemical differentiation of the Earth and in the transport of heat to the Earth surface. This special volume contains contributions presented at the symposium Physics and Chemistry of Partially Molten Systems' of the EUG 9 meeting, held in Strasbourg, France, on March 23-27, 1997. It is intended to provide a current understanding of the physics of partial melting and melt segregation and covers topics such as the rheology of partially molten systems, the topology of partial melts, modelling of partial melting processes, and field observations of partial melts. Audience: This book is intended for a broad readership, including graduate students, specializing in petrology and geodynamics. The volume may be recommended as a textbook for graduate courses on petrology, geomaterial sciences and geophysics.

It is an honour and pleasure to write a foreword to this useful and interesting book. Authors are very well known researchers who pioneered percolation modelling of transport in porous media in Russia from the early 80-th till nowadays. The main scope of the work presented in the book was developed when bright papers by A. Aharony, H.T. Davis, F.A.L. Dullien, A.A. Heiba, R.G. Larson, R. Lenormand, M.Sahimi, L.E. Scriven, D. Stauffer, M. Yanuka, Y.C.Yortsos were not available at the "other" side of the Iron Curtain. Nowadays hundreds of works and papers with the "percolation" keywords ap pear in petroleum and related applied research areas. The book will take a re markable place in the "petroleum percolation" bibliography. There are two important features of novelty in the monograph presented. First of all the authors developed a generalization of percolation clusters theory for grids with varying conductivity. Technique of representation of an infinite cluster as an hierarchial set of trees (so called r-chain model) allows to present conductivity of a stochastic grid in a closed form of explicit formulae. This method differs from those known in the West, such as effective media theory, solutions for the Bethe-lattice, etc. It has his own area of successful appli cations."

2Gpa has increased to more than 15. This indicates that subduction of continental fragments to depths of 100-150 km may have played a significant role in the formation of mountain belts. This volume brings together the geochemical, geophysical and geodynamical approaches to study the processes active during ultrahigh-pressure (UHP) tectonics. The collection of papers demarkates the frontier of our understanding of the creation, preservation, and exhumation of ultrahigh-pressure rocks. Audience: This volume will be of interest to any earth scientist interested in ultrahigh pressure processes and the formation and modification of continental crust.

1. Kimberlites and Orangeites.- 1.1. Etymology of Group I and II Kimberlites.- 1.2. Definitions of Cryptogenic and Primary Phases.- 1.3. The Hybrid Nature of Kimberlites and Orangeites.- 1.4. Philosophy and Principles of Classification.- 1.4.1. Modal versus Genetic Classifications.- 1.4.2. Petrological Clans.- 1.4.3. The Lamprophyre Clan.- 1.4.4. Mineralogical-Genetic Nomenclature within Petrological Clans.- 1.5. Mineralogical Comparisons between Kimberlites and Orangeites.- 1.6. Definitions of Orangeites and Kimberlites.- 1.6.1. Orangeites.- 1.6.2. Kimberlites.- 1.7. Age and Distribution of Orangeites.- 1.8. Occurrences of Orangeites.- 1.8.1. Finsch.- 1.8.2. Barkly West Region.- 1.8.2.1. Bellsbank.- 1.8.2.2. Sover.- 1.8.2.3. Newlands.- 1.8.2.4. Pniel.- 1.8.3. Boshof District.- 1.8.3.1. Roberts Victor.- 1.8.3.2. New Elands.- 1.8.4. Winburg District.- 1.8.5. Kroonstad District.- 1.8.6. Swartruggens District.- 1.8.7. Dokolwayo.- 1.8.8. Prieska District.- 1.8.9. Summary.- 1.9. Textural-Genetic Classifications of Petrological Clans....- 1.9.1. Kimberlites.- 1.9.1.1. Crater Facies.- 1.9.1.2. Diatreme Facies.- 1.9.1.3. Hypabyssal Facies.- 1.9.1.4. Spatial Relationships between Diatreme and Hypabyssal Facies Kimberlites.- 1.9.2. Orangeites.- 1.9.3. Melilitite Clan.- 1.10. Petrographic Characteristics of Orangeite.- 1.11. Petrographic Differences with Respect to Kimberlites.- 1.12. Petrographic Differences with Respect to Lamproites.- 2. Mineralogy of Orangeites.- 2.1. Mica.- 2.1.1. Paragenesis.- 2.1.2. Composition of Primary Mica.- 2.1.2.1. Al2O3-TiO2 Variation.- 2.1.2.2. Al2O3-FeOT Variation.- 2.1.2.3. Macrocrysts versus Microphenocrysts.- 2.1.2.4. Minor Elements.- 2.1.2.5. Trace Elements.- 2.1.3. Aluminous Mica-Microxenoliths.- 2.1.4. Aluminous Biotite Macrocrysts.- 2.1.5. Micas from the Swartruggens Male Lamprophyre.- 2.1.6. Summary of Mica Compositional Variation.- 2.1.7. Solid Solutions in Orangeite Mica.- 2.1.8. Mica in Kimberlites.- 2.1.8.1. Macrocrysts.- 2.1.8.2. Primary Micas.- 2.1.8.3. Summary of Kimberlite Mica Compositional Variation.- 2.1.9. Mica in Lamproites.- 2.1.10. Mica in Minettes.- 2.1.11. Mica in Ultramafic Lamprophyres.- 2.2. Clinopyroxene.- 2.2.1. Paragenesis.- 2.2.2. Composition.- 2.2.2.1. Diopside.- 2.2.2.2. Titanian Aegirine.- 2.2.2.3. Minor Elements.- 2.2.3. Pyroxenes in the Swartruggens Male Lamprophyre..- 2.2.4. Megacrystal Pyroxenes.- 2.2.5. Comparison with Pyroxenes in Kimberlites.- 2.2.6. Comparisons with Pyroxenes in Lamproites.- 2.2.7. Comparisons with Pyroxenes in Ultramafic Lamprophyres.- 2.2.8. Comparisons with Pyroxenes from Minettes.- 2.3. Olivine.- 2.3.1. Paragenesis.- 2.3.2. Composition.- 2.3.3. Comparisons with Olivines in Kimberlites.- 2.3.4. Comparisons with Olivines in Lamproites.- 2.4. Spinel.- 2.4.1. Paragenesis.- 2.4.2. Composition.- 2.4.3. Comparisons with Kimberlite Spinels.- 2.4.4. Spinel Compositional Variation in Lamproites and Lamprophyres.- 2.5. Potassium Barium Titanates.- 2.5.1. Hollandite.- 2.5.1.1. Paragenesis.- 2.5.1.2. Composition.- 2.5.1.3. Comparison with Hollandites from Lamproites, Kimberlites, and Other Potassic Rocks.- 2.5.2. Potassium Triskaidecatitanate.- 2.5.3. Barium Pentatitanate.- 2.6. Perovskite.- 2.6.1. Paragenesis.- 2.6.2. Composition.- 2.6.3. Comparison with Perovskites from Kimberlite.- 2.6.4. Comparison with Lamproite Perovskite.- 2.7. Phosphates.- 2.7.1. Apatite.- 2.7.1.1. Paragenesis.- 2.7.1.2. Composition.- 2.7.1.3. Comparison with Kimberlite and Lamproite Apatite.- 2.7.2. Daqingshanite.- 2.7.3. Monazite.- 2.7.4. Sr-REE Phosphate.- 2.8. Amphiboles-Potassium Richterite.- 2.8.1. Paragenesis.- 2.8.2. Composition.- 2.8.3. Comparison with Potassium Richterite in Lamproite and Other Potassic Rocks.- 2.9. Potassium Feldspar.- 2.10. Ilmenite.- 2.10.1. Comparison with Groundmass Ilmenites from Kimberlites.- 2.10.2. Comparison with Ilmenites in Lamproites.- 2.11. Rutile.- 2.12. Zirconium Silicates.- 2.12.1. Zircon.- 2.12.2. Wadeite.- 2.12.3. Zirconium-Bearing Gar

Feldspar minerals make up 60% of the crust of the Earth. They are stable in the upper mantle, and are so abundant in the crust that they form the basis of the classification of igneous rocks. At the surface, feldspars weather to form clay minerals which are the most important mineral constituent of soils. The articles in this book review the chemical reactions of feldspars over the whole sweep of pressure and temperature regimes in the outer Earth, and describe the fundamental aspects of crystal structure which underlie their properties. The book covers intracrystalline reactions, such as order-disorder transformations and exsolution, and transfer of stable and radiogenic isotopes, which can be interpreted to provide insights into the thermal history of rocks. It is suitable for final year undergraduates or research workers.

The fundamental principles of tephrochronology were de- veloped in Iceland through the classic research of Professor Sigurdur Thorarinsson in the 1940's and 1950's. By his studies on the volcanic ash layers (tephra) produced by the historic explosive eruptions of the volcano Hekla, he established that individual tephra layers could be correlated over large areas of Iceland. As the deposition of such layers was essentially instantaneous on a geological time-scale, the tephra layers provided a distinctive and widespread isochronous stratigraphic marker. Since Thorarinsson's pioneering work, the study of tephra has become a powerful and increasingly important research tool in many branches of the geological sciences and related disciplines. In response to the recent rapid growth and diversifica- tion of tephra studies we decided to coordinate a NATO Advanced Studies Institute under the auspices of the INQUA Commission on Tephra. The ASI, entitled "Tephra Studies as a Tool in Quaternary Research," was held from June 18th to June 29th, 1980, at Laugarvatn in Iceland. This book represents the proceedings of that Institute and is the first book dedicated to tephra studies, its many uses and applications. The subject matter combines disciplines as diverse as Quaternary strati- graphy, isotope geochronology, petrology, deep-sea geology, volcanology, volcanic hazards mapping, archaeology and ecology in one volume, and illustrates both present uses and future potential of tephra research.

The transport of heat through a porous medium in the presence of exterior forces, generally produced by the Earth's gravitational field and/or a pressure gradient, is called conduction when the Darcean fluid is static (motionless), and convection when the Darcean fluid is in motion. It is customary to use the term convection also to describe the motion which arises from the density differences due to temperature gradients within the Darcean fluid. We think that because this last phenomenon is more general it should be given a specific name; here we call it thermal flow. In the sense of the above definitions, convection and thermal flow are two distinct phenomena (they occur together, in underground combustion for instance), and the convective motion which arises when a Darcean l'luid is in contact with a source of heat is a particular case of thermal flow. Thermal flow occurs naturally and is important in many geophysical and industrial problems, particularly in oil exploration, and in the petroleum, chemical and nuclear industries (for instance, in the evaluation of capability of heat-removal from a hypothetical accident in a nuclear reactor). It can play a part in the transfer of heat from the deep interior of the Earth to a shallow depth in the geothermal regions. However, in the field of energy conversion little attention has yet been paid to the insulating characteristics of the saturated porous materials introduced in some enclosures (storage tanks) to decrease the convective and radiative transfer of heat.

Recently, many fine textbooks in mineralogy have notable early achievements in unravelling the crys- appeared. The great tradition of mineral science tal structures of minerals. I would include Zachari- continued for over 100 years in Dana's systems, as en and Belov as well, two other giants in crystal textbooks, and manuals, replete with discussion on structure analysis. How many contributed in their crystallographic characters, and short statements own way to our science? Ten? Fifty? One hundred? on the mineral species as then known. The more re- It depends on how you weight the count, but I cent superb RamdohrlStrunz Klockmann's Lehr- suspect it is closest to the biggest number just men- tioned. buch der Mineralogie is a mine of rich crystal- chemical information, perhaps the finest contem- The explosive growth in information (Ger. : porary pedagogic book of its kind. Within the past Fach) on crystal structures began about 1950, when three years, a new and ambitious project - the the big computers and automated diffractometers were just emerging. It contributes mightily to our Handbook of Mineralogy by several keen mineralo- gists - receives much assistance from up-to-date science (Ger. : Naturwissenschaft). Unfortunately, computer technology and promises to be a very fine technological knowledge seems to come easily now, series. The little Mineral Reference Manual by Nic- and Fach appears to be outstripping Naturwissen- schaft, a sorry state of affairs.

The feldspars form the most abundant group of minerals in the crust of the Earth and Moon and also occur in many meteo rites. They playa fundamental role in all rock-forming processes at shallow depths, but are rare or absent from the upper mantle. Their detailed study is thus essential for the understan ding of such varied processes as magma genesis and differentia tion, metamorphism, al teration, erosion and sedimentation. This interest is show by the fact that two previous NATO Advanced Study Institutes on feldspars were held in Oslo in 1962 and in Manchester in 1972. The feldspars are particularly sui table for detailed studies, as they have very simple chemistry and develop some of the most complex microstructures known. The microstructures are often slow to form but are easily preserved, so that they are potentially extremely informative about the geological history of the rocks in which they occur. Furthermore, their study involves physical and chemical methods of increasing sophistication so that the results obtained are not always immediately understandable to research workers outside the field of modern mineralogy. Progress in knowledge about feldspars is probably slower in penetrating the fields of petrology and geochemistry than that on other mineral groups. For these reasons among others, i was particularly appropriate to hold a third NATO ASI on feldspars approximately ten years after the last one."

Featuring over 250 contributions from more than 100 earth scientists from 18 countries, The Encyclopedia of Igneous and Metamorphic Petrology deals with the nature and genesis of igneous rocks that have crystallized from molten magma, and of metamorphic rocks that are the products of re-crystallization associated with increases in temperature and pressure, mainly at considerable depths in the Earth's crust. Entries range from alkaline rocks to zeolite facies - providing information on the mineralogical, chemical and textural characters of rock types, the development of concepts and the present state of knowledge across the spectrum of igneous and metamorphic petrology, together with extensive lists of both commonly used and little used terms and bibliographies.

For much of the 20th century, scientific contacts between the Soviet Union and western countries were few and far between, and often super ficial. In earth sciences, ideas and data were slow to cross the Iron Curtain, and there was considerable mutual mistrust of diverging scient ific philosophies. In geochemistry, most western scientists were slow to appreciate the advances being made in the Soviet Union by os. Korz hinskii, who put the study of ore genesis on a rigorous thermodynamic basis as early as the 1930s. Korzhinskii appreciated that the most fun damental requirement for the application of quantitative models is data on mineral and fluid behaviour at the elevated pressures and temper atures that occur in the Earth's crust. He began the work at the Institute of Experimental Mineralogy (IEM) in 1965, and it became a separate establishment of the Academy of Sciences in Chernogolovka in 1969. The aim was to initiate a major programme of high P-T experimental studies to apply physical chemistry and thermodynamics to resolving geological problems. For many years, Chernogolovka was a closed city, and western scient ists were unable to visit the laboratories, but with the advent of peres troika in 1989, the first groups of visitors were eagerly welcomed to the IEM. What they found was an experimental facility on a massive scale, with 300 staff, including 80 researchers and most of the rest pro viding technical support."

"Rock Mass Classifications - A Practical Approach in Civil Engineering" was written in response to the many unanswered questions regarding this subject. Questions such as - Is Classification reasonably reliable? Can it be successful in crisis management of geohazards? Can a single Classification system be general for all rock structures? Is Classification a scientific approach? Laborious field research was undertaken in the Himalayan mountains by a team of scientists from the Central Mining Research Institute (CMRI), University of Roorkee (UOR), Central Soil and Material Research Station (CSMRS), U.P. Irrigation Research Institute (UPIRI), and Norwegian Geotechnical Institute (NGI) to answer these questions. The results obtained from the research work were systematically compiled to produce this book which bears particular relevance to civil, mining and petroleum engineers and geologists.

Endorsements
"It is a Handbook of Rock Engineering" - Zhao Jian, School of Civil & Structural Engineering, Nanyang Technological University, Singapore
"I came across your new book - Rock Mass Classification, absolutely fantastic" - Subodh K. Jain, U.S.A

The study of multiphase flow through porous media is undergoing intense development, mostly due to the recent introduction of new methods. After the profound changes induced by percolation in the eighties, attention is nowadays focused on the pore scale. The physical situation is complex and only recently have tools become available that allow significant progress to be made in the area. This volume on Multiphase Flow in Porous Media, which is also being published as a special issue of the journal Transport in Porous Media, contains contributions on the lattice-Boltzmann technique, the renormalization technique, and semi-phenomenological studies at the pore level. Attention is mostly focused on two- and three-phase flows. These techniques are of tremendous importance for the numerous applications of multiphase flows in oil fields, unsaturated soils, the chemical industry, and environmental sciences.

The main aim of this paper is to present some new and general results, ap plicable to the the equations of two phase flow, as formulated in geothermal reservoir engineering. Two phase regions are important in many geothermal reservoirs, especially at depths of order several hundred metres, where ris ing, essentially isothermal single phase liquid first begins to boil. The fluid then continues to rise, with its temperature and pressure closely following the saturation (boiling) curve appropriate to the fluid composition. Perhaps the two most interesting theoretical aspects of the (idealised) two phase flow equations in geothermal reservoir engineering are that firstly, only one component (water) is involved; and secondly, that the densities of the two phases are so different. This has led to the approximation of ignoring capillary pressure. The main aim of this paper is to analyse some of the consequences of this assumption, especially in relation to saturation changes within a uniform porous medium. A general analytic treatment of three dimensional flow is considered. Pre viously, three dimensional modelling in geothermal reservoirs have relied on numerical simulators. In contrast, most of the past analytic work has been restricted to one dimensional examples."