178 HISTORY OF GEOLOGY FROM 1780 TO 1835 cooling and contraction as the major cause of mountain formation In essence, cooling/contraction theories held the field until the arrival of ‘drift’ and plate tectonic ideas in the twentieth century Minerals, Rocks, and Crystals Werner and his school concentrated on the study of the external features of minerals, and developed elaborate schemes for their description and classification The late eighteenth century saw the emergence of chemistry successfully applied to minerals The older methods of pyro-analysis with the help of the blowpipe, though useful in the field, could give little quantitative information But in Sweden Torbern Bergman (1784) published a general method for the chemical analysis of gems They could be brought into solution by fusion with alkali and then, by a sequence of precipitation reactions, and heating and weighing the several products, the different constituent ‘earths’ (silica, magnesia, alumina, lime, etc.) could be ascertained as percentages Bergman’s results were inaccurate, but the principles of his procedure were valid and were soon applied more successfully by chemists such as Richard Kirwan, Nicholas Vauquelin, Martin Klaproth, and Jons Jacob Berzelius, both to minerals and rocks Aided by Lavoisier’s theory of elements as simple substances, obtained as the last terms of chemical analysis, mineralogy had a satisfactory theoretical and practical basis for chemical understanding But old problems remained A substance of one chemical composition could have many different mineral forms and substances of similar crystalline form could have numerous different chemical compositions The question of the best way to characterize mineral species remained contentious Geology per se did not take a great leap forward through the progress in chemical mineralogy before 1830 In petrology, the distinction between bedding and cleavage was understood by the English geologist Adam Sedgwick by the 1820s, but he probably learnt it from quarrymen Following Ami Boue´ (1819), the category of metamorphic rocks was introduced by Lyell (1833): ‘altered stratified’ rocks—the alteration being due to heat and pressure He referred to ‘hypogene’ (formed-at-depth) rocks, instead of ‘primary’ or ‘primitive’, and he divided them into those that were ‘unstratified’ (plutonic, e.g., granite) and ‘stratified’ (metamorphic, e.g., gneisses or schists) In crystallography, the most important contributions came from the Frenchman Rene -Juste Hauă y, the Englishmen William Wollaston, William Whewell, and William Miller, and the German Eilhard Mitscherlich Hauă y (1784, 1801, 1822) supposed that crystals were made up of a small number of fundamental ‘mole´ cules inte´ grantes’ (tetrahedron, triangular prism, and parallelipipedon), which could be revealed by crystal cleavage and the ‘conceptual analysis’ of crystals From these starting points, he hypothesized the ‘building’ of many different crystals forms from similar basic building blocks, according to assumed rules of decrement for the addition of the ‘integrant molecules’ His reasoning was in part circular, but it gave intelligibility to crystallography Hauă ys integrant molecule foreshadowed the modern chemical concept of molecule Hauă y used contact goniometers, which were of limited accuracy Wollaston (1809) devised the more accurate reflecting goniometer, and its increased precision led him to question Hauă ys methods and results But Whewell (1824), developing Hauă ys concepts, was able to use co-ordinate geometry to describe crystals, arriving at the equations x/h þ y/k þ z/l ¼ or px þ qy þ rz ¼ m to represent crystal faces, all coefficients being integers The indices p, q, and r are now known as the Miller indices, being reciprocally related to the co-ordinates of a vector perpendicular to the plane of a crystal face By such analysis, crystallography could become mathematized and quantifiable, while geology remained in an ‘historical’ and largely qualitative mode Mitscherlich was responsible for introducing the concepts of isomorphism, dimorphism, and polymorphism, which assisted understanding of the complexities of empirical mineralogy Volcano Theory Chemistry also offered ideas about the Earth’s internal heat With the discovery of the alkali metals by Humphry Davy (1807), the suggestion was made that the heat might be generated by the action of water penetrating into subterranean stores of these metals, sufficient to produce volcanic eruptions This accorded with the idea that volcanoes might be produced by the expansion of gases within the Earth, causing localized ‘swellings’ of the crust (theory of ‘craters of elevation’ as advocated by Alexander von Humboldt and Leopold von Buch) There was extensive controversy concerning this issue, but Lyell’s theory of volcanoes being produced by successive accumulation of lava flows (or ash emissions) eventually prevailed Chemical theories of the Earth’s heat gradually declined in the nineteenth century, but improved suggestions were not really forthcoming until the twentieth century See Also Biblical Geology Famous Geologists: Cuvier; Darwin; Darwin; Hutton; Lyell; Sedgwick; Smith; Steno; Suess History of Geology Up To 1780