238 Mr. HOPKINS, ON THE TRANSPORT OF ERRATIC BLOCKS. 



it will become exposed to all the action of denuding agents, which however will, in many 

 instances, make less impression on those parts where the covering of detritus is thickest, or is 

 composed of the coarsest materials. Such parts will therefore, cceteris paribus, emerge first 

 from beneath the surface of the ocean ; and thus, in the first instance, will form islands, and 

 subsequently, when the whole shall have risen above the level of the sea, the summits of hills. 

 Such summits may consequently be expected to be capped with transported materials, of which 

 all traces may have been destroyed by denuding agents in the surrounding valleys. This pheno- 

 menon, of such frequent occurrence, is thus simply accounted for according to this theory. 



24. It appears by the table above given, (Art. 21) that a wave of between 50 and 100 feet 

 in height, (in an ocean of the original depth there supposed), would be accompanied with a 

 current of which the velocity would be from 10 to 20 miles an hour ; and it is demonstrated 

 in the first section, that (under conditions which I conceive to be entirely admissible) currents 

 of that velocity would possess a motive power abundantly sufficient to move the largest blocks, 

 the transport of which it would be deemed necessary to refer to this cause. But I would 

 particularly direct the attention of the reader to the fact, as exhibited in the values of s, in 

 the table just referred to, that the space through which a block may be transported by a 

 single wave, is equal only to a small fraction of the breadth of the wave. Consequently, a 

 great number of waves might be necessary for the transport of blocks to distances to which 

 they frequently have been transported. It must also be recollected, that sudden or paroxismal 

 elevations only will produce waves of elevation of considerable transporting power. Hence it 

 follows that this theory of transport is essentially and necessarily associated with that theory 

 which regards the phenomena of elevation as the consequences of a series of paroxismal move- 

 ments, the movements by which, in my opinion, those phenomena can be most satisfactorily 

 accounted for. The instantaneous elevation of a determinate portion of the bottom of the sea 

 would produce a wave whose height would be equal to that of the elevation itself, so that 

 it may be asserted in general terms, that the theory of transport by elevation currents, in its 

 application to existing phenomena of transport, involves the hypothesis of a succession of paro- 

 xismal movements beneath the ocean, the height of many of which must have varied from 50 to 

 100 feet at least. 



25. If we allow the efficiency of each of the three recognized means of transport of 

 erratic blocks — glaciers, floating ice, and currents — the difficulty which remains is that of sepa- 

 rating the effects produced by these causes respectively. In some cases it is probable that 

 doubt will always remain from insufficiency of evidence, but in others, I conceive, our conclu- 

 sions may involve but little uncertainty. The distinctive characters in the transported materials 

 must be sought in the magnitude and form of the blocks, the state of their surfaces, and the 

 distribution of the general mass of the transported materials. The magnitude of a block can 

 hardly be considered to increase the difficulty of its transport by ice, while it increases in a great 

 degree the difficulty of transport by water. Again, blocks cannot generally be rounded by 

 attrition when floated on icebergs or carried on the upper surface of a glacier. A small portion of 

 those brought down by glaciers are rounded by being rolled between the ice and sides or bottom 

 of the glacial valley ; but this is a rough grinding, and all the specimens I recollect to have 

 examined immediately at the termination of a glacier, wanted that more perfect smoothness of 

 surface which distinguishes a water-worn boulder. It might be contended that blocks floated 

 on icebergs might be rounded and polished before being taken up by the ice or after being 

 deposited by it. If such were the case, the effects must be produced either on beaches by the 

 action of breakers, or at the bottom of the sea by that of currents. The action of breakers, 

 on large blocks, however, as far as my observation has extended, rarely tends to give to them 

 a rounded form, but, on the contrary, to wear them into very irregular shapes, till they are 

 so reduced in magnitude as to be rolled about by the force of the waves ; the most prominent 



