On Drift. 15 



whole process of deposition of the finer matter, each of these pro- 

 cesses occupying distinct and separate intervals of time. No one 

 would, of course, suppose that the matter reposing on a given sur- 

 face of striated rock could have heen deposited there before that 

 surface became striated ; but the real question is, whether these 

 two processes of striating and depositing were not going on simul- 

 taneously in the region generally, though not absolutely the same 

 points. If the stride be due, as some geologists have supposed, to 

 detrital matter driven by a rapid current, the two processes must 

 of necessity have been simultaneous, the one where the current 

 was most rapid, the other where it was less so. Or if we refer the 

 striae in the lower and natter regions of the area of the drift to 

 floating ice, how was it that the icebergs and the currents which 

 impelled them onwards bore no detrital matter at that time, and so 

 much at a subsequent time 1 I conceive the two processes to 

 have gone on simultaneously. No agency for the production of 

 striated and polished surfaces has ever yet been suggested which 

 would not almost necessarily be accompanied with the transport, and 

 consequently with the deposition of detrital matter. Currents and 

 small icebergs might deposit from time to time detrital matter on a 

 given rock- surface, but the first iceberg that succeeded, large enough 

 to reach down to that surface and grind over it, would clear away 

 the detritus previously deposited upon it, and smooth and striate 

 the rock itself. This might be repeated for a long period of time, 

 during which the process of striating the projecting surfaces might 

 be contemporaneous with that of permanent deposition at points 

 almost immediately contiguous, but at lower levels. Finally, sup- 

 posing a continued subsidence of the general area, the projecting 

 striated bosses would sink below the reach of the icebergs, and the 

 transport of matter still continuing, would become permanently 

 covered up. As the general area re-emerged it would be subject 

 to denudation, which might be expected to lay bare again some of 

 the striated rocks, and leave others permanently covered with de- 

 trital matter as we now find them. 



Again, with reference to the combined operations of floating ice 

 and currents, it is not unworthy of remark that the former would 

 necessarily deposit least of its freight, cceteris paribus, in its unim- 

 peded motion over deeper waters, and a greater part in its impeded 

 course over shallow bottoms. On the contrary, currents would 

 deposit least on the shallow bottoms, where, cceteris paribus, their 

 velocity would be greatest, and most in the deeper waters ; and, 

 moreover, it would be in these deeper waters that the finer matter 

 would be deposited. Thus the existence of beds of finer and in 

 many cases stratified deposits, having more tumultuous deposits, 

 possibly both above and below them, as in some parts of North 

 America, does not necessarily indicate a cessation in the more en- 

 ergetic action of the forces of dispersion, but may merely indicate 



