Page:Encyclopædia Britannica, Ninth Edition, v. 10.djvu/227

Rh GEOLOGY and habitat enable us to realize what their forerunners were. It has often been insisted upon that the present is the key to tile past ; and in a wide sense this assertion is eminently true. - Only in proportion as we understand the present, where everything is open on all sides to the fullest investi- gation, can we expect to decipher the past, where so much is obscure, imperfectly preserved, or not preserved at all. A study of the existing economy of nature ought thus to be the foundation of the geologist’s training. While, however, the present condition of things is thus employed, we must obviously be on our guard against the d mger of unconsciously assuming that the phase of 11ature’s operations which we now witness has been the same in all past time, that geological changes have taken place in former ages in the manner a11d on the scale which we behold to—day, and that at the present time all the great geological pro- cesses, which have produced changes in the past eras of the earth's history, are still existent and active. Of course we may assume this uniformity of action, and use the assump- tion as a working hypothesis. But it ought not to be allowed any ﬁrmer footing, nor on any account be suffered to blind us to the obvious truth that the few centuries wherein man has been observing nature form much too brief an interval, by which to measure the intensity of geological action in all past time. For aught we can tell the present is an era of qnietnde a11d slow change, compared with some of the eras which have preceded it. Nor can we be sure that, when we have explored every geological process now in p1'0gress, we have exhausted all the causes of change which, even in comparatively recent times, l1ave been at work. In dealing with the Geological Record, as the accessible solid part of the globe is called, we cannot too vividly realize that at the best it forms but an imperfect chronicle. Geological history cannot be compiled from a full and con- tinuous series of documents. From the very nature of its origin the record is necessarily fragmentary, and it has been further mutilated and obscured by the revolutions of suc- cessive ages. And even where the chronicle of events is contiuuoas, it is of very unequal value in different places. In one case, for example, it may present us with an un- broken succession of deposits many thousands of feet in thickness, from which, however, only a few meagre facts as to geological history can be gleaned. In another instance it brings before us, within the compass of a few yards, the evidence of a most varied and complicated series of changes in physical geography, as well as an abundant and interest- ing suite of organic remains. These a11d other character- istics of the geological record will become 111ore apparent and intelligible as we proceed in the study of the science. In the systematic treatment of the subject the following arrangement will here be followed :— l. The C'osnu'c(Ll Aspects of G'eulo{/_y.—Untler this head we may consider the evidence supplied by astronomy and physics regarding the form and motions of the earth, the composition of the sun and planets, and the probable history of the solar system. G'eo,r/1z0s_r/,—an Inq2u'7'3/ into I/ze J[az‘cr2'al.s of the Earl/L’s S'ub.stI(nI_‘€.—I1) this division we deal with the parts of the earth, its c11vclopes of air and water, its solid crust, and the probable condition of its i11terior. Especially, we have to study the more important minerals of the crust, and the chief rocks of which that crust is built up. I11 this way we lay a foundation of knowledge regarding the nature of the materials constituting the mass of the globe, and may next proceed to investigate the processes by which these materials are produced and altered. 3. l)_2/nmnz}-al Geo[og_2/ embraces an investigation of the various agencies whereby the rocks of the earth’s crust are formed and metamorphosed, and by which changes are 213 effected upon the distribution of sea and land, and upon the forms of terrestrial surfaces. Such an inquiry neces- sitates a careful study of the existing geological economy of nature, and forms a ﬁtting introduction to the investi- gation of the geological changes of former periods. This and the previous section include n1ost of what is embraced under Physical Geography; and for the reason stated under that heading the subject will here be treated n1ore in detail than is usual in geological treatises. 4. Structural Geologg/, or the Arc/u'tcclm'e of (Ice Earl/t.—— We now advance to consider how the various materials composing the crust of the earth have been arranged. We learn that son1e have been formed in beds or strata on the floor of the sea, that others have been built up by the slow aggregation of organic forn1s, that others have been poured I out in a molten condition or in showers of loose dust from subterranean sources. We further find that, though origin- ally laid down in almost horizontal beds, the rocks have subsequently been crumpled, contorted, and dislocated, that they have been incessantly worn down, and have often been depressed and buried beneath later accumulations. 5. 1’alceonlologz'cal Geology. —This branch of the subject deals with the organic forms which are found preserved in the crust of the earth. It includes such questions as the relations between extinct and living types, the laws which appear to have governed the distribution of life in time and in space, the relative importance of different genera of animals in geological inquiry, the nature and use of the evidence from organic remains regarding former conditions of physical geography. This subject will be 1nore properly discussed in the article PAL.»EoNToLo(:v, and will therefore be only cursorily treated in the following pages. 6. .S'lrat2'grap/z{cal G'colo_r/y. —Tl1is section might be called geological history. It works out the chronological succes- ,sion of the great formations of the eartl1’s crust, and endeavours to trace the sequence of events of which they contain the record. More particularly it determines the order of succession of the various plants and animals which in past time have peopled the earth, and thus ascertains what has been the grand n1arcl1 of life upon the planet. 7. I’/1_ysi0gr((p/tical Geology, starting from the basis of fact laid down by stratigraphical geology regarding former geographical changes, embraces an inquiry into the origin and history of the features of the earth’s surfaee——contin- ental ridges and ocean basins, plains, valleys, and moun- tains. It explains the causes 011 which local differences of I scenery depend, and shows under what very different cir- 4 I cumstances, and at what widely separated intervals, the hills and mountains, even of a single country, have been produced. PART I.—COS.[ICAL ASPECTS OF GEOLOGY. Before geology had attained to the position of an induc- tive science, it was customary to begin all investigations into the history of the earth by propounding or adopting some more or less fanciful hypothesis in explanation of the origin of our planet, or even of the universe. Such pre- liminary notions were looked upon as essential to a right understanding of the manner in which the materials of the globe had been put together. To the illustrious James Hutton (1785) geologists are indebted for strenuously up- holding the doctrine that it is no part of the province of geology to discuss the origin of things. He taught them that in the matcri-a.ls from which geological evidence is to be compiled there can be found “ no traces of a beginning, no prospect of an end.” In England, mainly to the influence of the school which he founded, and to the subse- quent rise of the Geological Society (1807), which resolved to collect facts instead of ﬁghting over hypotheses, is due