TBANSACTIONS OF SECTION C. 699 



of tliese three life-stages in tbe history of a mechanical formation. No present 

 geological processes ai'e better known to the young geologist than those of 

 denudation, erosion, and transportation, so familiar to us in the eloquent works of 

 our President. They form together the subject-matter of that most wonderful and 

 fascinating chapter in geology which, from its modest opening among the quiet 

 Norfolk sandhills, sweeps upwards and onwards without a break to its magnificent 

 close on the brink of the gorge of the Colorado. But our knowledge of the de- 

 tailed processes of deposition and consolidation which rule in the second stage 

 is still exceedingly imperfect, although a flood of light has been thrown upon 

 the subject by the brilliant results of the Challenger Expedition. And we are 

 compelled to admit that our knowledge of the operations of those agencies which 

 rule in the processes of upheaval and depression is as yet almost nil ; and what 

 little we have already learnt of the effects of those agencies is the prey of hosts of 

 conflicting theories that merely serve to annoy and bewilder the working student 

 of the science. 



But not one of the formative triad of detrition, deposition, and elevation can 

 exist without the others. No detrition is possible without the previous upheaval 

 of the rock-sheet from which material can be removed ; no deposition is possible 

 without the previous depression of tbe rock-sheet which forms the basin in which 

 the fragmentary material can be laid down. 



Our knowledge, therefore, of the origin and meaning of any geological 

 formation whatever can at most be only fragmentary until this third chapter in 

 the life-history of the geological formation has been attacked in earnest. 



Now, all the rich store of knowledge that we possess respecting the first stage 

 in the life of a geological formation has been derived from a comparison of the 

 phenomena which the stratigraphical geologist finds in the rock formations of the 

 past, with correspondent phenomena which the physical geographer discovers on 

 the surface of the earth of the present. And all that we know of the second stage 

 has been obtained in precisely the same way. Surely analogy and common sense 

 both teach us that all which is likely to be of permanent value to us as regards 

 the final stage of elevation and depression must first be sought for in the same 

 direction. 



Within the last twenty years or so many interesting and vital discoveries have 

 been made in the stratigraphy of the rock formations that bear largely upon this 

 obscure chapter of elevation and depression. And I propose on this occasion that 

 we try to summarise a few of these new facts ; and, reading them in conjunc- 

 tion with what we actually know of the physical geography of the present da,y, try 

 to ascertain how such mutual agreement as we can discover may serve to aid the 

 stratigraphical geologist in his interpretation of the true meaning of the geological 

 formations themselves. We may not hope for many years to come to read the 

 whole of this geological chapter, but we may perhaps modestly essay an interpre- 

 tation of one or two of the opening paragraphs. 



In the physical geography of the present day, we find the exterior of our 

 terraqueous globe divided between the two elements of land and water. We know 

 that the solid geological formations exist everywhere beneath the visible surface of 

 the lands, but of their existence under the present ocean floor we have as yet no 

 absolute certainty. We know both the form of the surface and the composition 

 of the outer layers of the continental parts of the lithosphere ; we only know as 

 yet even in outline the form of the surface of its oceanic portions. 



The surface of each of our great continental masses of land resembles that of 

 a long and broad arch-like form, of which we see the simplest type in the New 

 World. The surface of the North American arch is sagged downwards in the 

 middle into a central depression which lies between two long marginal plateaux ; 

 and these plateaux are finally crowned by the wrinkled crests which form its two 

 modern mountain systems. The surface of each of our ocean floors exactly resembles 

 that of a continent turned upside down. Taking the Atlantic as our simplest type, 

 we may say that the surface of an ocean basin resembles that of a mighty trough 

 or synchne, buckled up more or less centrally into a medial ridge, which is bounded 

 by two long and deep marginal hollows, in the cores of which still deeper grooves 



