394 



GEOLOGY. 



on this subject by ancient writers. We shall only 

 here notice the principal modem theories. 



According to the sacred theory of Burnet, the 

 whole materials of which the earth is composed were 

 united together in one fluid chaotic mass. When 

 these elements began to separate, the heavier parti- 

 cles formed a nucleus, and the water and the air 

 occupied places according to their specific gravity. 

 A crust formed of matters collected from the air was 

 deposited upon the water, and afforded a station for 

 numerous tribes of vegetables. No seas, or moun- 

 tains, or inequalities, were seen on the surface of the 

 globe, which was then clothed with the richest irer- 

 dure, but the power of the sun producing cracks and 

 fissures in the external crust, the surface was broken 

 up and destroyed, and the fragments sinking into the 

 abyss of waters occasioned the universal de-luge ; the 

 waters again retiring into caverns and fissures, the 

 dry land appeared, and islands and continents were 

 formed as they are now seen. 



In an essay towards a natural history of the earth, 

 by Woodward, it is supposed that a new crust was 

 deposited on the earth from the waters of the deluge, 

 and that the materials of which it is composed are 

 arranged according to their specific gravity ; the 

 heaviest and hardest bodies forming the nucleus and 

 being covered by those of a finer and lighter quality. 



Buflbn's theory was, that the planets in general 

 were struck off from the sun by a comet ; and that, 

 being composed of fluid matter, they assumed a 

 spherical form, and, by the operation of centrifugal 

 and centripetal forces, were retained in their orbits. 

 As the earth cooled, the surrounding vapours were 

 condensed on its surface, and other matters finding 

 their way into fissures and cavities, formed veins and 

 masses of metallic and earthy minerals. But by the 

 motion of the earth, by the action of the sun, winds, 

 and tides, new changes were produced. The waters 

 were greatly elevated about the equator, and brought 

 with them solid fragments from the polar regions. 

 The surface of the globe now exhibited a broken and 

 irregular aspect ; land arose in one place and exca- 

 vations were formed in another. 



In Werner's theory it is assumed, that the materi- 

 als of which the external crust of the earth is com- 

 posed were either dissolved or suspended in water ; 

 and that the first class of rocks were deposited from 

 a state of chemical solution, and thus exhibit a 

 crystallized appearance. In the next series of rocks 

 a few organized remains are observed, and hence it 

 is concluded that marine animals were created about 

 the time when these rocks, which are called transi- 

 tion rocks, were formed. After this period the waters 

 of the earth holding in suspension particles of matter 

 in a state of minute division, derived from the disin- 

 tegration of the first series of rocks, began to subside 

 more rapidly, and to deposit that series of strata 

 which are denominated secondary rocks or floetz 

 rocks, because they are deposited in beds in a hori- 

 zontal position. By the action of the air and the 

 operation of water, the three classes of rocks being 

 wasted down and broken, produced inequalities on 

 tlie surface of the earth, arid the waters still farther 

 subsiding deposited the different kinds of alluvial 

 matters. In this system a fifth class of rocks, in- 

 cluding those substances which are ejected by vol- 

 canoes, are called volcanic rocks. The formation 

 of vertical strata forms a prominent feature in theo- 

 ries of the earth. According to the theory of Wer- 

 ner, as the waters whicli held in solution or suspension 

 the materials of which the solid parts of the earth 

 are composed subsided, fissures were formed, and the 

 waters holding in solution other earthy and metallic 

 matters, again covered the earth and deposited these 

 mattirs in the fissures. 



Button's theory is, that the surface of tlie globe 

 is in a state of constant destruction and renovation. 

 By the action of air and water the hardest rocks are 

 subject to decay and decomposition ; and the disin- 

 tegrated materials are conveyed to the ocean, and 

 there accumulating, are formed into horizontal layers. 

 The strata thus deposited are supposed to be conso- 

 lidated by the heat of central fires ; and to the action 

 of the same subterraneous heat, the production of the 

 primitive rocks, which are supposed to have been 

 projected in a state of fusion from the bowels of the 

 earth, ;is ascribed. To the operation of the same 

 cause, all kinds of basaltic rocks are also supposed 

 to owe their existence ; and the materials of metallic 

 veins, and of the vertical strata, have been ejected 

 in a state of fusion from the centre of the earth and 

 deposited in fissures either previously formed, or 

 which they form for themselves in their progress 

 through the superincumbent strata. In this theory 

 the operation both of fire and water is introduced ; 

 the disintegration and decomposition of the solid 

 parts of the globe are produced by water, and, being 

 deposited at the bottom of the ocean, are consolidated 

 and hardened by heat ; and a new series of rocks, 

 projected in a state of fusion by the action of the 

 same power, burst through the secondary strata, and 

 elevating it to great heights, constitute a new series 

 of primitive rocks. This system, then, holds out 

 a perpetual system of renovation and decay ; or, as 

 it is assumed and expressed by Dr Hutton himself, in 

 the series of changes which are constantly going on, 

 " there is no symptom of a beginning and no prospect 

 of an end." See Playfair's Illustrations of the Hut- 

 tonian Theory, Edin. 1802, 8vo. 



In order to understand more fully these theories, 

 let us now consider the destroying and the form- 

 ing effects of water and fire on the surface of the 

 <arth. 



Destroying and forming effects of water. Water 

 acts either chemically or mechanically on the surface 

 of the earth. Every long continued rain convinces us 

 of the powerful mechanical effects of water on the sur- 

 face of the earth. The precipitated water penetrates 

 the surface, then flows along mixed with the matter of 

 the soil, and in its course forms small water-courses, 

 and occasions considerable changes in the flat country 

 and the declivities of mountains. As it rains a very 

 considerable portion of the year, and as every rain 

 carries along with it a quantity of the soil, very 

 obvious changes must in this manner be induced on 

 the surface of the globe. Thunder-storms and water- 

 spouts, although more uncommon phenomena, pro- 

 duce more considerable changes, and this eithei 

 alone, or when their waters join or flow into rivers. 

 These deprive whole districts of their soil to the bare 

 rock ; they sometimes even form small ravines, and 

 break down and carry away great masses of rock, 

 that were either formerly much rent, or of such a 

 form as to be easily overpowered by water. If such 

 changes take place in the low land, they must be 

 vastly more considerable in the high land. 



The thaw-floods that take place in low countries 

 towards the end of winter and beginning of spring, 

 and in mountainous districts during summer, occasion 

 still greater changes on the surface of the earth. 

 Their effects are truly frightful, particularly when 

 accompanied with rain. The declivities in low coun- 

 tries, over which water flows, are less considerable 

 than in high countries ; and besides, the water can 

 extend itself farther in low and flat countries ; hence 

 its destroying effects are diminished in intensity. In 

 mountainous countries, on the contrary, the fall is 

 much greater than in flat countries, and the water is 

 compressed into narrow rocky valleys ; hence it fol- 

 lows, that rain-floods must be more destructive, the 



