On Drift. 5 



the great work of transport according to the characters of the 

 transported materials ; for it is alone by a careful study of these 

 distinctive characters that we can hope to decide by what agent 

 the transport has been effected. On this point there appears to 

 be still much discrepancy of opinion, when the test has to be 

 applied to individual cases. These differences of opinion seem to 

 manifest themselves principally on questions relating to the action 

 of water, either with reference to the form in which currents tend 

 to deposit a general mass of drift, or to their effect in rounding and 

 wearing the individual component parts of it, as compared with the 

 tendency of other modes of transport to produce similar effects. 

 It may be that we have not yet studied these effects as referable to 

 different causes with sufficient care, or that we are still too much 

 influenced individually by preconceived notions ; but it is certain 

 that different persons do draw very different inferences as to the 

 mode of transport of a given mass of drift, from the characters 

 which its component materials present. In some cases such in- 

 ferences will probably ever remain doubtful, but in others there 

 can be no reasonable grounds for doubt. Most geologists appear 

 now to agree about what may be regarded as the two extreme 

 cases, and admit small rounded pebbles as a proof of long-con- 

 tinued aqueous action, and very large erratics with perfectly un- 

 worn angles as equally indicative of transport by ice. If there 

 be any among us not glacialists to this extent, I recommend them 

 to the personal study of these blocks. I well recollect, in my 

 own case, that after resisting all verbal arguments in favour of 

 glacial theories, I stood at once convinced under the silent appeal 

 of the Pierre ct bot on my visit to that magnificent erratic of the 

 Jura. In almost all the cases intermediate to these extremes, I 

 fear we have much yet to reconcile before we come to any unity 

 of opinion. And here, gentlemen, let us ask ourselves in the 

 spirit of candour, whether one cause of this may not be found in 

 our natural tendency to hold too pertinaciously to preconceived 

 opinions. It will not be denied by any one, I imagine, that it 

 would generally be the necessary consequence of a transitory cur- 

 rent driving a mass of drift over a level surface, to spread it out in 

 an approximately equable layer; while such a result could generally 

 be regarded as only the accidental consequence of transport by 

 floating ice. Suck a layer would indicate the latter as a possible 

 mode of deposition, the former as a highly probable one. When 

 the glacialist contends for the possible rather than the probable 

 mode, let him examine himself strictly whether he may not be 

 unconsciously under the dominion of preconceived theoretical 

 views. Again, the polishing of rocks and their striation in de- 

 finite directions may be generally regarded as the necessary con- 

 sequences of the passage over them of a large mass of ice, pre- 

 serving its general direction of motion in defiance of merely local 



