of climatic geomorphology that has been credited as a contributing factor for the decline in the popularity in field the late 20th century. Widely cited version of the Varnes system of landslide classification first introduced in 1958. Geophysics A classic reference on the Earth's magnetic field and related topics in meteorology, solar and lunar physics, the aurora, techniques of spherical harmonic analysis and treatment of periodicities in geophysical data. Its comprehensive summaries made it the standard reference on geomagnetism and the ionosphere for at least 2 decades. Geotechnical engineering The book is regarded as the "basis of modern soil mechanics". Inaugurated "the fundamental differential equation necessary to start the procedure of consolidation" and "the theory of effective stresses" that "justify the way in which the soil behaves under load". Hoek, E. (1994) "Strengths of rock and rock masses" ISRM News Journal, 2(2): 4–16. Introduced the Geological Strength Index. Hydrogeology The work contains the first determination of Darcy's law. Inferred the fact that both "water and the porous structure" of an aquifer are "elastically compressible". The work establishes the well function, permitting calculations of transmissivity and storability and from aquifer test data under ideal conditions. Mineralogy and petrology Systematic treatise of then known minerals and gemstones as well as other rocks, the first since Pliny's Natural History. "[T]he first modern textbook on mineralogy." Influential work in petrography. The first mapping of "progressive metamorphism through the use of mineral isograds". Established the concept of metamorphic facies. Originally published in 1928, it applied the principles of chemistry to petrological problems. Considered a benchmark paper in the study of fluid inclusions. Considered a benchmark paper in the study of coal petrology. Demonstrated experimentally that granite can crystallize from felsic melt. Results are presented in a chemical-mineralogical ternary diagram. A condensation of Rock-Forming Minerals (1962), a 5-volume comprehensive treatise of the physical, chemical, mineralogical, petrological and optical properties of essentially all minerals with nontrivial abundances to be found in terrestrial rocks. Also presents information regarding common origins and associations of each mineral, as well as a practical commentary on how to distinguish each mineral from others which may appear similar. First outline of the classification of granite into I and S types. One of the first outlines of the International Union of Geological Sciences classification of plutonic rocks. Established the TAS classification of volcanic rocks. Igneous Petrogenesis has long been a key reference and advanced