10 On Drift. 



deposition in a deeper sea. If also, large angular blocks from dis- 

 tant sites should be imbedded in this mass of finer matter, we see 

 an additional indication of a deep sea, in which a floating iceberg 

 would, perhaps, at distant intervals, drop a portion of its freight. 



There is also a consideration connected with the process of 

 transport by certain currents alone, which, with reference to our 

 inferences as to the succession of events, is of some importance. I 

 have mentioned it in my memoir " On the Granitic Blocks of the 

 South Highlands of Scotland," which appears in the last Number 

 of our Journal. Currents attending waves produced by sudden 

 elevations, greater or less, are necessarily transitory, and each can 

 only carry the materials it may transport to certain distances, de- 

 pending, cceteris paribus, on the magnitudes of the component in- 

 dividual masses, the large blocks being carried but to small dis- 

 tances, and the smaller particles to much greater distances, t Thus 

 the first wave would produce a layer consisting of the larger blocks 

 near their source and of fine detritus at the remoter distances. The 

 second wave would produce a similar effect, and would also carry 

 the blocks of the first wave to a somewhat greater distance, and so 

 on for successive waves. The effect, then, of a succession of simi- 

 lar waves would be the formation, over the more remote parts of 

 the area of deposition, of a bed of finer matter, in the upper por- 

 tion of which would exist blocks rounded and waterworn by their 

 transit. Thus we should have the phenomena of fine detrital matter 

 below and blocks above, apparently referable to several successive 

 periods of time, during the first of which one kind of agency should 

 have transported the finer sediment, and during the second another 

 and much more powerful agency should have transported the blocks 

 and coarser detritus, while, in fact, the whole phenomena would be 

 really referable to a repetition of precisely the same agency during 

 the whole period of transport. That period, therefore, except in a 

 limited sense, and not with reference to the whole area of trans- 

 port, could not, in the case now supposed, be divided into two, but 

 must be regarded as one single period. 



I do not mean here to assert the opinion that the actual glacial 

 period recognised by geologists was characterised by a uniform suc- 

 cession of exactly similar events producing erratic dispersion. 

 There might be particular portions of that period in which acci- 

 dental circumstances produced a greater or less prevalence of each 

 particular mode of transport ; but I am satisfied that some of the 

 attempts which have been made to subdivide the glacial period 

 have been made without due regard to such considerations as those 

 which I have given above. 



Let us now turn again to the drift of North America. The 

 American geologists appear for the most part to recognise three 

 distinct periods into which the whole period of the drift may be di- 

 vided. The first period was one of the transport of blocks and 



