GEOLOGY, in the broadest sense, has for its object the elucidation of the history of the earth and its living inhabitants. For practical purposes, however, it is necessary to adopt somewhat arbitrary limitations. The earliest stages in the development of the earth belong to astronomy and cosmogony, whereas, at the other end of the scale, geology in some of its aspects merges into history and geography, as well as into the biological sciences, in cluding anthropology and ethnology. It may perhaps be said that the province of geology begins at the period when the earth ac quired a permanent solid crust ; from that time onward it studies the development of the physical features of the earth, the com position and structures of the rocks composing it, and the evolu tion of animals and plants from their unknown beginnings. It will thus be seen that geology is a very comprehensive subject, closely linked to numerous other sciences, and capable of being regarded from many different points of view, but for most purposes it can be divided more or less clearly into four distinct branches, or five if, as is usual, the cosmogonic aspect is included.
1. Cosmogony deals with the relation of the earth to the solar system and to the universe, its origin and early stages.
2. Physical Geology is the study of the forms of the earth's surface, their structure, their manner of origin, and the nature of the processes that have moulded and are still modifying it.
3. Petrology describes the composition of the rocks that now form the accessible portion of the crust, and their mode of origin.
4. Palaeontology may be defined as the biology of fossils, the remains of former animals and plants now buried in the rocks.
5. Stratigraphical Geology works out the history of the earth and its geography in past ages through the study of the rocks and the fossils that they contain.
The present article includes sections dealing with the second and fifth of these categories; Cosmogony, Petrology and Palaeon tology are relegated to separate articles.
I. HISTORY OF THE SCIENCE The earliest beginnings of geological thought, like those of many other sciences, are so mixed up with myth and legend and vague cosmical speculations, and so little founded on observa tion and induction, that a lengthy consideration of them would not be very illuminating from the scientific point of view, though of much interest in the development of philosophy and psychology. However, even these myths and legends must have had their basis in the observation and interpretation of natural phenomena, so that they are in some sense geological.
It so happens that the Mediterranean basin, the home of many of the early civilizations whose written records have come down to us, is an area in which geological phenomena are peculiarly well displayed and striking, and it may well be a matter of wonder that the acute thinkers of antiquity did not manage to arrive at a more reasonable and more correct interpretation of the facts than they did : it is perhaps worth remembering that geology even in its modern forms depends far less on complicated instruments and appliances than almost any other science. It is nearly all naked-eye observation of things on a large scale, and therefore might have been thought peculiarly well adapted to development at an early stage of civilization.
Many of the earlier philosophers of Greece and Rome were naturally dissatisfied with the vague and often obviously im possible theories of the meaning of natural phenomena embodied in the popular myths and traditions, and sought to find more reasonable explanations of them. The opinions entertained in antiquity on these subjects may be conveniently grouped under two heads : geological processes in operation at the time, and (2) geological changes in the past.
1. Contemporary Processes.-Earthquakes, owing to their often disastrous character, received much attention from the Greeks and Romans. Aristotle, in his Meteorics, quotes the views of older writers and rejects them in favour of his own idea that earthquakes are due to wind within the earth, produced by the warmth of the sun and internal heat. He looked upon earthquakes and volcanic phenomena as closely connected, the eruption being due to the final escape of the wind that caused earthquakes. Very similar and equally crude ideas were put forward by Lucre tius, who also followed Anaximenes in attributing earthquakes in part to falls of rock within the earth. Strabo recognized that Vesuvius was a dormant volcano, though there was then no record of an eruption ; he continued to believe in wind as the active agent in underground disturbances. He cites in his Geography a number of cases of sinking and rising of land and he recognized that some islands are volcanic, while others were originally part of the mainland. The most detailed account of earthquakes that has come down to us from antiquity is in the Quaestiones Naturales of Seneca. He was much interested in an earthquake at Naples in A.D. 63 and distinguished several different motions of the ground, up and down, oscillatory and vibrational. He held firmly to the old idea that the most important factor was wind, while admitting the possibility of some shocks being due to collapse of caverns. Volcanoes he supposed to be due to this wind meeting stores of sulphur and other combustibles, which were set on fire by friction.
The action of rivers also, being of much importance to mankind, was observed with more or less care. Herodotus noted the great amount of silt brought down annually by the Nile, and per tinently remarked "Egypt is the gift of the river," which is per fectly true. Aristotle displays considerable knowledge of the drainage system on the north side of the Mediterranean basin. He refers to the mountains as condensers of rain and snow and shows that the largest rivers rise in the highest ground. He also points out how river deposits are encroaching on the sea, and mentions specially in this connection some parts of the Black Sea, where of ter sixty years it became necessary to use ships of much lighter draught, as the water had become much shallower and would soon be converted into dry land. Strabo also discussed the formation of plains and deltas by rivers.
From these selected examples it might appear that the classical writers had accumulated a good deal of observational material, but in point of fact most of the conclusions that they drew as to ultimate causes were mere speculation and guesswork. It can hardly be said that scientific geology made any real progress throughout these ages.
After the general revival of classical studies, the problems of geology, as dealt with in the ancient writings, once more began to attract attention, especially in Italy. In that country fossil shells are very abundant, and several different theories of their origin were put forward. Some writers regarded them as mere freaks of nature "lapides sui generis," which had never been alive ; others considered them to be relics of the universal deluge. More practicable theories soon began to prevail. Leonardo da Vinci (1452-1519) maintained that they had once been normal living organisms, and that what is now land had formerly been covered by the sea. Girolamo Frascatorio (1483-1553) claimed that a single temporary deluge was inadequate to produce the observed effects, but that the mountains in which the shells are found must have been uplifted from the sea.
But it was the Catholic ecclesiastic, Nicholas Steno (Stensen) (1631-87) at one time Bishop of Hamburg and Vicar-Apostolic of Denmark, afterwards resident in Florence, who must be re garded as the founder of modern geology and mineralogy. After making a world-wide reputation as an anatomist, he turned to the study of fossils, and in 1669 published a small tract with the not very lucid title De solido inter solidunt naturaliter contento in which he developed his ideas as to the past history of the earth as derived from his study of the rocks, dealing with many of the fundamental problems of physical geology and stratigraphy on quite modern lines.
The Italian geological school of this period also included An tonio Vallisneri (1661-173o) who had a wide knowledge of fossils, and Lazzaro Moro (1687-174o) who made an extensive study of stratigraphy but attached too much weight to volcanic accumula tion as the source of sedimentary deposits.
The Cosmogonists and Their Successors. In other countries however, and especially in England, the study of geology ran on rather different lines. Instead of a patient accumulation of facts and inferences from those facts alone many writers indulged in fanciful speculation and wildly extravagant theories of cos mogony, mostly founded on sheer imagination. During the last twenty years of the 17th century there appeared a number of writings which attracted attention mainly by their sensationalism, both in England and abroad. As examples may be mentioned the works of Thomas Burnet and William Whiston. Hardly less fanciful in his theories, although a much better observer, was John Woodward whose Essay towards a Natural History of the Earth appeared in 1695. His large and valuable collection of fossils is still religiously preserved intact in the Sedgwick Museum at Cambridge, in accordance with the terms of his will. Wood ward published a catalogue of this collection in 1728-29 under the title An attempt towards a Natural History of the Fossils of England. It is to be noted that at this time the term fossil in cluded minerals as well as organic remains and the Woodwardian Collection contains some very fine mineral specimens.
A striking contrast is afforded by another group ,of geologists in France and Germany, who gave to the world the first really practical ideas as to the evolution of the earth. The first of them was Rene Descartes (1596-165o) who ascribed the origin of the earth and the other planets to the cooling of an originally incan descent mass like the sun, leading to the formation of a solid crust over a still hot nucleus.
The ideas of Descartes were enlarged and systematized by Wilhelm Gottfried Leibnitz (1646-1716) whose work has had a permanent value. In his great tract, the Protogaea, published only in 1749, he traced the passage of the earth from an incan descent mass of vapour to a molten globe, which by further cooling acquired a solid crust, perhaps now surviving in part as some of the most ancient rocks. He thought that as cooling went on great cavities might be left, filled with air and water; these cavities by collapse might give rise to valleys, while the more solid parts formed the mountains. Water escaping from the cavities might form the sedimentary deposits. Leibnitz also attached much importance to fossils as indices of past changes and alternations of land and sea.
Of much greater importance however in the development of geology was the work of G. L. L. de Buffon (1707-88) who had a much wider acquaintance with rocks and fossils. He accepted the ideas of Descartes and Leibnitz as to planetary evolution, but in his first geological work (1749) he supposed that fossils had all been deposited in a universal ocean, which was later swallowed by the cavities within the earth. Thirty years later, in his famous Epoques de la Nature, he endeavoured to draw up a chronology of the earth, dividing it into six periods of unequal length, based on the supposed rate of cooling from the incandescent stage.
To return for a while to the French school, it may perhaps be said that the founder of invertebrate palaeontology was Lamarck who is universally regarded as one of the originators of the doctrine of evolution and continuous progress of living beings, as opposed to the theory of catastrophic destruction and repeated creations, which largely prevailed everywhere during his time. This eminent naturalist turned his attention somewhat late in life to the study of fossils, which led him to formulate some geological ideas very similar to those of Hutton and Playfair, though arrived at quite independently. However, he did not pay sufficient attention to the possibilities of earth-movement, and attributed even the highest non-volcanic mountains to the wearing away of elevated plateaux, and he attached an exaggerated value to the tides as geological agents. As a biologist he brought fossils into the forefront of his theories as a means of working out the history of the earth and the succession of events.
In a somewhat similar way the great comparative anatomist, Cuvier (1769-1832) is to be regarded as the founder of verte brate palaeontology. Cuvier's knowledge of comparative anatomy was so marvellous that he was literally almost able to recon struct a whole animal from a single bone or tooth. In a preliminary section of his great work, Recherches sur les ossemens fossiles (1821), he gave an outline of what he believed to have been the history of the globe. This was frankly catastrophic, in the most extreme form of the doctrine ; he believed that there was evidence for the occurrence in the past of cataclysms of world-wide extent, far surpassing even the conventional destructiveness of the Biblical deluge ; he rejected entirely all ideas of organic continuity and postulated fresh creations of new forms of animals and plants. It is interesting, to digress for a moment, to find that after a century of the quietistic Uniformitarianism of Hutton and Lyell, there are now signs of a return to a form of catastrophism, though with out a complete destruction of life.
During the latter half of the eighteenth century and the begin ning of the nineteenth there was a violent and rather absurd con troversy as to the origin of the igneous rocks; some writers con tinued to maintain, up to the middle of the eighteenth century, that volcanic phenomena are due to combustion and they attrib uted lava flows to the melting of ordinary rocks, such as clay and sandstone, by the heat of such combustion. The true nature o vulcanicity will be discussed in detail later : it must suffice to sa; here that eruptions are due to the escape, under gas pressure, o portions of the heated interior of the earth in a molten state giving rise to flows of lava and showers of ashes, with steam am other vapours. But in earlier days trouble arose over the nature 0; certain rocks found among the earlier formations while resembling the products of modern volcanoes, and also as to the origin o such rocks as granite which appear to have invaded newer forma tions while in a molten condition.
The last and most famous of the geologists of this older schoo was Abraham Gottlob Werner (1749-1815) of Freiberg, Saxony Werner apparently was a magnificent teacher, although the doctrines he taught were erroneous, and he certainly deserves thf utmost credit as the real founder of economic geology. Th( Mining Academy at Freiberg has been in the forefront of thf study of the metalliferous deposits ever since his day, but Werner'5 obstinate adherence to the worn-out doctrine of the aqueous origir of basalt had a disastrous effect on petrographical geology foi many years.
As early as 1752 J. E. Guettard (1715-86) had studied the extinct volcanoes of Auvergne and had shown that the basaltic lava, there so abundant, far from being a chemical precipitate, as supposed by Werner, was in reality poured out in a molten state at many different times and places. Desmarest (17 2 5-1815) fol lowed this up a few years later by mapping the area in detail and working out the whole complicated history. In Scotland Hutton proved the intrusive nature of granite and various "trap" rocks. J. F. d'Aubuisson, who had written a memoir on the aqueous origin of the basalts of Saxony, was converted by a visit to Auvergne, and even the great Leopold von Buch, at one time the most shining light of the Neptunist school, abandoned his Wernerian faith for the same reason. From that time the igneous origin of both deep seated intrusions and surface lava-flows was established.
From about this same date, the beginning of the nineteenth century, the history of the development of geology becomes so complicated that it is impossible to follow up all the threads simul taneously, or within any reasonable compass to give a general account of the whole. The various branches, such as physical geology, petrology, palaeontology and stratigraphy, began to have a more or less clearly defined separate existence, each with its own line of evolution and development, under the impulse of varying forces, such as the advance of physics and chemistry, the coming of evolution, the advance of geographical exploration and so forth, and all these various lines of progress ceased to be confined to one small corner of north-western Europe, but spread over the world. Hence from this point onward the only possible course is to exercise some kind of selection. The development of petrology and palaeontology will be sufficiently apparent from the articles on those subjects, while the closely allied subject of mineralogy has a history of its own—troubled by far fewer controversies. What follows, therefore, is a brief sketch of the progress of gen eral geology, especially on its physical and stratigraphical sides, in the British Isles during the nineteenth century, with occa sional excursions into other parts of the world where needed by the context.
One of the most remarkable figures in the whole history of geology is William Smith (1789-1839) a man of quite humble origin, who spent most of his life as a canal engineer in Somerset and as a land-agent near Scarborough. During a busy professional life he managed to find time to construct a geological map of England and Wales, and at the same time to work out the suc cession of the stratified formations of the country by means of their contained fossils. The majority of the rather rough-and ready names that he coined for the different formations have re mained to this day as the classical designations, and it is to be hoped that they may so continue to remain, a reminder to the stratigraphers of all countries of the birthplace of their science. William Smith has aptly been called the "Father of British Geology," and his large coloured wall-map, dated 1815, compares to some extent with the latest productions of the Geological Sur vey and was a monumental achievement for one self-taught man. The work of the pioneers of physical and stratigraphical geology was admirably generalized and amplified by Sir Charles Lyell (1797-1875). He was able to travel extensively throughout his life, and to add by his own acute observations great masses of facts to the stores accumulated by his predecessors. His Principles of Geology (3 vols., 1830-33), which rapidly passed into numerous editions, is one of the great books of the world. On the physical side the line of thought is strictly Huttonian, an exposition of the doctrine of uniform causes, while on the biological side for many years Lyell maintained the permanence of species; but in later life he frankly became a convert to the doctrine of evolution, especially the form of it expounded by Darwin. It is a question which of these great men was the more influenced by the other; the whole tendency of Lyell's work was in the direction of con verting geology from a physical into a biological science, and he took little interest in mineralogy, apart from volcanic phenomena. Lyell's Principles remains a classic of geology, and may still be profitably read. About the same time the work of G. P. Scrope in Auvergne, following on the lines laid down by Guettard and Desmarest, established the modern science of vulcanology.
Up to the first quarter of the nineteenth century very little was known about the older rocks constituting the western and the northern parts of Great Britain, though by 1830 the succession of the newer formations in the south and east had been fairly thoroughly worked out. About this time Adam Sedgwick (1785 1873), Professor of Geology at Cambridge, and Sir Roderick Impey Murchison (1792-1871) began to investigate the rocks of Wales and the western Midlands. Sedgwick started in the north west in Caernarvonshire and succeeded in working out the suc cession of the slaty rocks far to the south-east of his starting point, calling them the Cambrian system. Murchison on the other hand began in Shropshire and worked north-westwards, naming his rocks the Silurian system. In general terms Sedgwick was working upwards in the succession of the stratified rocks and Murchison downwards ; but the sequence was not quite simple. The rocks there are thrown into wavy folds, by crumpling of the crust, so that there is some repetition of strata at the surface. The natural result was that the respective systems of Sedgwick and Murchi son overlapped. Murchison claimed for his Silurian system nearly the whole of Sedgwick's Cambrian, leaving him only a remnant of unfossiliferous slates at the bottom, while Sedgwick on the other hand perhaps claimed too much for his Cambrian. Lapworth pro posed as a practical compromise that the beds mostly in dispute (Murchison's Lower Silurian) should be constituted into a separate system, called the Ordovician. This is now universally accepted, though for many years Sir Archibald Geikie (1835-1924) valiantly upheld the Lower Silurian.
The brilliant and original work of Charles Lapworth (1842 i920) exercised a profound influence on geology in the third quarter of the nineteenth century. He established the order of succession of the different forms of the great group of fossils known as the graptolites, which were found to follow one another in a definite evolutionary order, each in a layer of rock (of very variable thickness in different places, be it understood), which Lapworth called the zone of that particular fossil. By means of these zones he was able to unravel the intensely complicated structure of the southern uplands of Scotland and of other areas, thus throwing a flood of light on mountain-building and the folding of the earth's crust. By thus revealing the "Secret of the Highlands" he paved the way for the proper understanding of the geology of many other areas. Lapworth's great work has been and still is being ably carried on by his pupils, his followers and their disciples. Another problem of first-class importance and some what cognate nature that arose during this same period was the question of the time order of succession of the rocks of the north west Highlands of Scotland and the age of the crystalline rocks composing the central and the southern Highlands. The intensely complicated structures of the far north-west were worked out by Lapworth and the officers of the Geological Survey, and proved to be the relics of an ancient mountain chain of Alpine type. In the central and southern Highlands work is still in progress.
From this point it becomes impossible to refer in any detail to the progress of geology without mentioning the names of living workers. Immense progress has been made in British stratigraphy, mostly in matters of detail, without any very fundamental new discoveries. In structural geology one of the most important advances is the recognition of the principle of "posthumous move ment," which means in simple language that when once the earth's crust has been crumpled into a fold running in a particular direc tion, further crumpling is likely to take place along the same line at later dates. This principle has proved of immense importance in working out the structure and limits of coalfields buried under newer strata. Further, the zonary method of stratigraphy has been extended to nearly all the fossiliferous formations.