August i8, 1892] 



NATURE 



373 



stratigraphical geology have universally acknowledged that in 

 the study of present geographical causes lies the key to the 

 geological formations and the inorganic world of the past. 



In this way the road was paved for Darwin and the doctrine 

 of descent. The aid which had been so ungrudgingly aft'orded 

 by biology to geology was repaid by one of the noblest presents 

 ever made by one science to another. For the purposes of 

 geology, the science of biology had practically completed a 

 double demonstration : first, that the extinct life discernible in 

 the geological formations was linked inseparably with the organic 

 life of the present ; and, second, that every fossil recognized by 

 the geologist was the relic of a creature that might well have 

 existed upon the surface of the earth at the present time. Geology 

 repaid its obligation to biology by the still greater twofold 

 demonstration : first, that in the economy of nature the most in- 

 significant causes are competent to the grandest effects, if only 

 a sufficiency of time be granted them ; and, second, that in the 

 geological formations we have the evidences of the actual 

 existence of those mighty eons in which such work might be 

 done. 



The doctrine of organic evolution would always have re- 

 mained a metaphysical dream had geology not given the time 

 in which the evolution could be accomplished. The ability of 

 present causes to bring about slow and cumulative changes in 

 the species is, to all intents and purposes, a biological applica- 

 tion of Hutton's ideas with respect to the original geological 

 formations. Darwin was a biological evolutionist, because he 

 was first an uniformitarian geologist. Biology is pre-eminent 

 to-day among the natural sciences, because its younger sister. 

 Geology, gave it the means. 



But the inevitable consequence of the work of Darwin and 

 his colleagues was that the centre of gravity, so to speak, of 

 popular regard and public controversy was suddenly shifted from 

 stratigraphical geology to biology. Since that day stratigraphi- 

 cal geology, to its great comfort and advantage, has gone quietly 

 on its way unchallenged, and all its more recent results have, at 

 least by the majority of the wonder-loving public, been practi- 

 cally ignored. 



Indeed, to the outside observer it would seem as if strati- 

 graphical geology for the last thirty years had been practically 

 at a standstill. The startling discoveries and speculations of the 

 brilliant stratigraphists of the end of the last century and first 

 half of the present forced the geology of their day into the very 

 front rank of the natural sciences, and made it perhaps the most 

 conspicuous of them all in the eyes of the world at large. Since 

 that time, however, their successors have been mainly occupied 

 in completing the work of the great pioneers. The strati- 

 graphical geologists themselves have been almost wholly occu- 

 pied in laying down upon our maps the superficial outlines of 

 the great formations, and working out their inter-relationships 

 and subdivisions. At the present day the young stratigraphical 

 student soon learns that all the limits of our great formations 

 have been laid down with accuracy and clearness, and finds but 

 little to add to the accepted nomenclature of the time. 



Our palasontologists also have equally busied themselves in 

 working out the rich store of the organic remains of the geo- 

 logical formations, and the youthful investigator soon discovers 

 that almost every fossil he is able to detect in the field has already 

 been named, figured, and described, and its place in the 

 geological record more or less accurately fixed. 



In France, in Germany, in Norway, Sweden, and elsewhere, 

 in Canada and in the United States, work as thorough and as 

 satisfactory has been accomplished, and the local development 

 of the great stratified formations and their fossils laid down with 

 detail and clearness. 



Many an unfledged, but aspiring geologist, alive to these 

 facts, and contrasting the well- mapped ground of the present 

 time with the virgin lands of the days of the great pioneers, 

 finds it hard to stifle a feeling of keen regret that there are now- 

 adays no new geological worlds to conquer, no new systems to 

 discover and name, and no strange and unexpected faunas to 

 unearth and bring forth to the astonished light of day. The 

 youth of stratigraphical geology, with all its wonder and fresh- 

 ness, seems to have departed, and all that remains is to accept, 

 to commemorate, and to round ofif the glorious victories of the 

 dead heroes of our science. 



But to the patient stratigraphical veteran, who has kept his 

 eyes open to discoveries new and old, this lull in the war of 

 geological controversy presents itself rather as a grateful breath- 

 ing time ; the more grateful as he sees looming rapidly up in 



front the vague outlines of those oncoming problems which it 

 will be the duty and the joy of the rising race of young geologists 

 to grapple with and to conquer, as their fathers met and 

 vanquished the problems of the past. He knows perfectly well 

 that Geology is yet in her merest youth, and that to justify even 

 her very existence there can be no rest until the whole earth- 

 crust and all its phenomena, past, present, and to come, have 

 been subjected to the domain of human thought and compre- 

 hension. There can be no more finality in Geology than in any 

 other science ; the discovery of to-day is merely the stepping- 

 stone to the discovery of to-morrow ; the living theory of to- 

 morrow is nourished by the relics of its parent theory of to-day. 



Now if we ask what are these formations which constitute the 

 objects of study of the stratigraphical geologist, I am afraid that, 

 as in the case of the species of the biologist, no two authorities 

 would agree in framing precisely the same definition. The 

 original use of the term formation was of necessity lithological,^ 

 and even now the name is most naturally applied to any great 

 sheet of rock which forms a component member of the earth- 

 crust ; whether the term be used specifically for a thin 

 homogeneous sheet of rock like the Stonesfield slate, ranging 

 over a few square miles ; or generically, for a compound sheet 

 of rock, like the Old Red Sandstone, many thousands of feet in 

 thickness, but whose collective lithological characteristics give 

 it an individuality recognizable over the breadth of an entire 

 continent. 



When Werner originally discovered that the *' formations " of 

 Saxony followed each other in a certain recognizable order, a 

 second characteristic of a formation became superposed upon 

 the original lithological conception — namely, that of determinate 

 "relative position." And when William Smith proved that 

 each of the formations of the English Midlands was distinguished 

 by an assemblage of organic remains peculiar to itself, there 

 became added yet a third criterion — that of the possession of 

 "characteristic fossils." 



But these later superposed conceptions of time-succession 

 and life-type are far better expressed by dividing the geological 

 formations into zoological zones, on the one hand, and grouping 

 them together, on the other hand, into chronological systems. 

 For in the experience of every geologist he finds his mind in- 

 stinctively harking back to the bare lithological application of 

 the word " formation," and I do not see that any real advantage 

 is gained by departing from the primitive use of the term. 



A zone, which may be regarded as the unit of palceonto- 

 logical succession, is marked by the presence of a special fossil^ 

 and may include one or many subordinate formations. A, 

 system, which is, broadly speaking, the unit of geological suc- 

 cession, includes many " zones," and often, but not always, 

 many "formations." A. formation, which is the unit of geo- 

 logical stratigi-aphy, is a rock sheet composed of many strata 

 possessing common lithological characters. The formation may 

 be simple, like the Chalk, or compound, like the New Red Sand- 

 stone ; but, simple or compound, local or regional, it must 

 be always recognizable, geographically and geologically, as a 

 lithological individual. 



As regards the natural grouping of these lithological indi- 

 viduals as such, fair progress has been made of late years, and 

 our information is growing apace. We know that there are at 

 any rate three main groups : 1st. The stratified formations due 

 to the action of moving water above the earth-crust ; 2nd. The 

 igneous formations which are derived from below the earth-crust ; 

 3rd. The metamorphic formations which have undergone change 

 within the earth-crust itself. We know also that of these three 

 the only group which has hitherto proved itself available for 

 the purpose of reading the past history of the globe is that of 

 the stratified formations. 



Studying these stratified formations therefore in greater detail, 

 we find that they fall naturally in their turn into two sets — viz., 

 a mechanical set of pebble beds, sandstones and clays formed of 

 rock fragments washed off the land into the waters, and an 

 organic set of limestones, chalk, &c., formed of the shells and 

 exuvice of marine organisms. 



But when we attempt a further division of these two sets our 

 classification soon begins to lose its definiteness. We infer that 

 some formations, such as the Old Red and the Triassic, were the 

 comparatively rapid deposits of lakes and inland seas ; that 

 others, like the Coal Measures, London Clay, &c., were the less 

 rapid deposits of lagoons, river valleys, deltas, and the like ; 

 that others, like our finely laminated shales and clays of the 

 Silurian and Jurassic, were the slower deposits of the broader 



NO. II 90. VOL. 4.61 



