The late eighteenth century saw the emergence of chemistry successfully applied to minerals.. But in Sweden Torbern Bergman 1784 pub-lished a general method for the chemical analysis of
Trang 1cooling 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
infor-mation But in Sweden Torbern Bergman (1784)
pub-lished 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
prod-ucts, the different constituent ‘earths’ (silica, magnesia,
alumina, lime, etc.) could be ascertained as
percent-ages Bergman’s results were inaccurate, but the
prin-ciples 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
sub-stances, 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
sub-stances of similar crystalline form could have numerous
different chemical compositions The question of the
best way to characterize mineral species remained
con-tentious 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
‘hypo-gene’ (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
contribu-tions came from the Frenchman Rene´-Juste Hau¨y, the
Englishmen William Wollaston, William Whewell, and
William Miller, and the German Eilhard Mitscherlich
Hau¨y (1784, 1801, 1822) supposed that crystals were
made up of a small number of fundamental ‘mole´cules
inte´grantes’ (tetrahedron, triangular prism, and paral-lelipipedon), which could be revealed by crystal cleav-age 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 crystal-lography Hau¨y’s ‘integrant molecule’ foreshadowed the modern chemical concept of molecule
Hau¨y used contact goniometers, which were of limited accuracy Wollaston (1809) devised the more accurate reflecting goniometer, and its increased preci-sion led him to question Hau¨y’s methods and results But Whewell (1824), developing Hau¨y’s concepts, was able to use co-ordinate geometry to describe crys-tals, arriving at the equations x/h þ y/k þ z/l ¼ 1 or
px þ qy þ rz ¼ m to represent crystal faces, all coeffi-cients 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, crystallog-raphy 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 in-ternal 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 pro-duced 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 exten-sive controversy concerning this issue, but Lyell’s theory of volcanoes being produced by successive accumulation of lava flows (or ash emissions) eventu-ally prevailed Chemical theories of the Earth’s heat gradually declined in the nineteenth century, but im-proved 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
178 HISTORY OF GEOLOGY FROM 1780 TO 1835