Nov. 20, 1873] 



NATURE 



51 



recognition of this fact has involved not a few of the 

 writers on this subject in great confusion and error. 



Among the geologists of the present day there is a 

 growing conviction that upheaval and subsidence are 

 concomitant phenomena, and that viewed broadly they 

 both arise from the effects of the secular cooling and con- 

 sequent contraction of the mass of the earth. The con- 

 traction has not been uniform, as if the globe had been a 

 cooling ball of solid iron. On the contrary, owing to very 

 great differences in the nature and condition of the various 

 parts of our planet and perhaps to features of the interior 

 with which we are yet but imperfectly acquainted, some 

 portions have sunk much more than others. These, 

 having to accommodate themselves into smaller dimensions 

 would undergo vast compression and exert an enormous 

 pressure on the more stable tracts which bounded them. 

 It could not but happen that after long intervals of strain, 

 some portions of the squeezed crust would at length find 

 relief from this pressure by rising to a greater or less 

 height, according to their e.xtent and the amount of force 

 from which they sought to escape. These upraised areas 

 would no doubt tend to occur in bands or lines across 

 the direction of the pressure, much as the folds we pro- 

 duce in the sheets of an unbound book are more or less 

 nearly parallel with the two sides from which we squeeze 

 the paper. They would sometimes be broad folds — huge 

 wide swellings of the earth's surface. At other times they 

 might be long, lofty, and comparatively sharp ridges. In 

 the one case they would give rise to high plateaux or 

 table-lands, in the other they would be recognised as 

 mountain-chains. 



This is a rough-and-ready statement of what seems the 

 probable explanation of the origin of the elevated tracts 

 upon the earth's surface. It is evident that the pressure 

 would be vastly gieater a few hundreds or thousands of 

 feet underground than at the surface, and hence that 

 though the rocks deep down might be squeezed and 

 crumpled, as we could crumple brown paper, yet that at 

 the surface they might show little or no contortion. Cer- 

 tainly without further proof we could never aflirm that a 

 contorted mass of rock which now forms the surface of 

 the ground rose as part of the surface during the time of 

 upheaval and contortion. Intensely crumpled rocks 

 would rather suggest a deeper position, with the subse- 

 quent removal of the rocks under which they originally 

 lay. 



As the earth has been cooling and contracting ever 

 since it had a separate existence as a planet, its surface 

 must have been exposed to a long series of such shrinkage 

 movements as those we are considering. Apart, there- 

 fore, from local evidence, we should expect that ridges 

 and depressions must have been impressed upon that sur- 

 face in a long succession from the earliest periods down- 

 wards, and hence that the present mountain-chains and 

 basins of the earth must be of many different ages. We 

 cannot tell what the first mountains were made of, nor 

 where they lay, although some of the existing ridges of 

 the earth's surface are undoubtedly, even in a geological 

 sense, very old. In not a few cases the same mountain- 

 chain can be shown from its internal structure to be of 

 many successive dates, as if it lay along a line of weak- 

 ness which had served again and again as a line of relief 

 from the severe earth-pressure. 



These questions have been treated with much ability 

 by Constant Prevost, Dana, Mallet, and others, to whose 

 writings I refer for details. In stating them in this 

 general way my object is to show that those geologists 

 who, like myself, believe in the truth of the Huttonian 

 doctrines of denudation, are most unfairly represented 

 when they are said to ignore the influence of subterranean 

 forces upon the exterior of the earth. None can recog- 

 nise more clearly than they do how entirely have the 

 great surface outlines of the globe been dependent upon 

 the action of these forces, that is, upon the results which 



flow from the contraction of the planet and from the re- 

 action of the heated interior upon the surface. 



But a block of marble is not a statue, nor would a part 

 of the earth's crust heaved up into land form at once such 

 a surface of ridge, and valley, and nicely adjusted water 

 system as any country of which we know anything on the 

 face of the globe. In each case it is a process of sculp- 

 ture, and the result varies not only with the tools but with 

 the materials on which they are used. You would not 

 expect the same kind of carving upon granite as upon 

 marble. And so, too, in the great process of earth- 

 sculpture, each chief class of rock has its own characte- 

 ristic style. The tools by which this great work has been 

 done are of the simplest and most everyday order — the air, 

 rain, frost, springs, brooks, rivers, glaciers, icebergs, and 

 the sea. These tools have been at work from the earliest 

 times of which any geological record has been preserved. 

 Indeed, it is out of the accumulitcd chips and dust which 

 they have made, afterwards hardened into solid rock and 

 upheaved, that the very framework of our continents has 

 been formed. The thickness of these consolidated mate- 

 rials is to be measured, not by feet merely, but by miles. 

 If the removed materials are so thick, they show what a 

 vast mass of rock must have been carved away. And 

 even before knowing anything of the way in which the 

 various tools are used, we should be justified in holding 

 it to be, at the least, extremely improbable that any land 

 surface would long retain its original contour or even any 

 trace of it. 



But when we come to watch with attention how the 

 tools really do their work, this improbabihty increases 

 enormously. Adopting a method of inquiry suggested 

 by Mr. Croll, I have elsewhere shown that even at their 

 present state of progress the amount of geological change 

 which they would accomplish in a comparatively small 

 number of ages is almost incredible. On a moderate 

 computation they would reduce the general mass of the 

 British Islands down to the level of the sea in five or 

 six millions of years, and might carve out valleys a 

 thousand feet deep in a fourth part of that time. It is 

 evident that though the upheaval of some parts of the 

 continents may go back into the remotest geological an- 

 tiquity, the forms of the present surface must be, com- 

 paratively speaking, modern. 



There is reason to believe that many, if not most, of 

 the great mountain chains of the globe are, in a geologi- 

 cal sense, of recent origin. The Alps, for example, 

 though they may have undergone many earlier move- 

 ments, were ridged up into their existing mass long after 

 the soft clays were laid down which cover so large an 

 area of the low lands in the south of England, and on 

 which London is built. It would require far more de- 

 tailed work than has ever been bestowed upon these 

 mountains to enable us even to approximate to what was 

 the original form of the surface just after the upheaval, 

 and before the array of sculpture-tools began their busy 

 and ceaseless task upon these great masses of rock. We 

 may believe that a series of huge parallel folds of curved 

 and broken rock rose for thousands of feet into the air, 

 that when, after the earth-throes had cea-ed, rain and snow 

 and frost first laid their fingers on the new-born summits, 

 these agents of destruction would have a most uneven 

 surface to work upon, and would necessarily be guided by 

 it in their working ; and hence that some, at least, of the 

 dominant eailiest ridges and hollows would be perpetu- 

 ated. Such a belief would cany probability in its favour, 

 but it would certainly not amount to a proof of the sup- 

 posed perpetuation. That would require to Idc corrobo- 

 rated by the internal and external evidence of the moun- 

 tains themselves. In some tracts, as, for instance, among 

 the singularly symmetrical ridges and furrows of the Jura, 

 it would not be difficult to restore the original outline, and 

 to fix exactly how far the subterranean movements had 

 determined the present external forms of the ground, 



