Mh. HOPKINS, ON THE TRANSPORT OF ERRATIC BLOCKS. 237 



SECTION III. 

 Application of' the preceding Theory. — Comparison of different Modes of Transport. 



22. In estimating tlie magnitude of a block which may be moved by a given current, the 

 transport has been supposed to take place over a horizontal surface, sufficiently hard and even 

 for the block to roll upon it without impediment. If the surface be otherwise constituted, 

 the motion may be impeded or destroyed. The softness of a clayey surface would probably 

 be most unfavourable to the motion ; while the want of cohesion of a sandy bottom, from its 

 opposing a less effective resistance to a motion rather by sliding than rolling, might be highly 

 favourable to the transport of the block. In any case a constant action of denuding causes 

 will be highly favourable to it, by the successive removal of temporary and local impediments. 

 Abrupt inequalities, such for instance as those presented by ravines and steep escarpments, 

 would present insuperable impediments to this mode of transport. It is important however 

 to observe, that regular ascents, without rugged inequalities of surface, would offer no such 

 serious impediment. 



The difficulty in this theory arising from the presumed inequalities of the surface over 

 which the blocks must have been transported, has been, I conceive, in many instances, far too 

 much insisted on; for it has been made to rest on the assumption that the inequalities of surface 

 between the present and original sites of erratic blocks were the same, or nearly so, at the 

 time of transport as at present; an assumption which I regard as totally untenable. There 

 are three obvious causes of inequality of surface — elevation and disruption, denudation during 

 gradual emergence from beneath the ocean, and erosion after emergence. So far as sudden, abrupt 

 inequalities can be traced to the first cause operating previously to the transport, the difficulty 

 alluded to must be admitted; but in many cases existing inequalities have been produced by 

 post-tertiary elevations, which we have no right to assume to have been entirely anterior to 

 the transport of erratic blocks. Again, such great inequalities as those presented by the oolitic 

 and chalk escarpments, have doubtless been due in a great measure to denudation, durinc the 

 period of gradual emergence of the land, the higher levels being raised above the sphere of 

 denuding action, while the lower levels remained exposed to it. Minor local irregularities of 

 surface are also due in a great degree to erosion. All superficial inequalities, therefore, which 

 are referrible to these causes, must have been posterior to the removal of erratic blocks trans- 

 ported by currents, and form no objection to that mode of transport. The only other causes 

 which can materially affect the configuration of the terrestrial surface, are the deposition of 

 new sedimentary beds, and denudation produced by ocean currents previously to any partial 

 emergence of the surface. But it is manifest that both these causes, instead of creating those 

 abrupt superficial inequalities, which alone would form a serious impediment to the transport we 

 are considering, must constantly tend to destroy them wherever they may exist from other causes. 

 For these reasons, I believe that there is no validity in the objection above stated to the theory 

 of transporting currents. Those greater superficial inequalities which now exist, and are obvi- 

 ously referrible to denuding agencies, could not, I repeat, be the consequences of superficial 

 denudation, while the whole surface was submerged beneath the ocean ; and minor abrupt inequa- 

 lities could not then have continued to exist, even if they had originally existed, for they would 

 have been destroyed by the action of transporting currents themselves, though no other cause 

 should have operated to produce that effect. 



23. These currents, in addition to tlicir transport of larger blocks, must manifestly tend to 

 spread out the smaller detritus in a layer over the bottom of the ocean, supposed, for the reasons 

 above stated, to form an even surface*. As the bottom rises in the process of slow elevation. 



In the iitniie for ioHtiiiice, in which ihc bottom of the (icnnan Ocean or English Channel in an even siirfaee. 



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