1851.] HOPKINS— ERRATIC BLOCKS OF THE S. HIGHLANDS. 29 



here concerned, is demonstrable, if we admit the existence of such 

 currents as must necessarily have resulted in this district from 

 elevatory movements, not much greater in magnitude than those 

 which have taken place during the historic period. I may also re- 

 mark, that in discussing this subject, we are too apt to direct our 

 attention exclusively to the phsenomena of what is emphatically 

 termed the Drift-period. It should be recollected that some of the 

 older geological periods furnish us also with records of similar phse- 

 nomena. The conglomerate of the Old Red Sandstone in the western 

 Highlands presents an enormous aggregation of boulders of consider- 

 able magnitude. In the neighbourhood of Oban, it is seen in imme- 

 diate juxtaposition with the erratic blocks of granite. The blocks of 

 the older period have evidently been subjected to very great attrition, 

 and have thus doubtlessly been much reduced below their original 

 size. Many of them, probably, were as large as the neighbouring 

 blocks of granite. Mr. J. C. Moore, also, in his paper " On the 

 Fossihferous Beds of Wigtonshire *," has described some very re- 

 markable beds of conglomerates containing, among smaller boulders, 

 rounded blocks of 4 or 5 feet in diameter, and regularly interstra- 

 tified with other beds. Mr. Griffith has also described Silurian con- 

 glomerates in Connamara containing blocks of great magnitude. It 

 would seem extremely difficult to assign the transport of these blocks 

 to any other agency than that of water. 



There are also phsenomena connected with the denudation of this 

 district which ought to be discussed in conjunction with the trans- 

 port of its erratic boulders. Most geologists will probably allow the 

 validity of the reasons above assigned for the opinion that this region 

 was submerged several hundred feet beneath the surface of the sea 

 during a portion of the glacial period, and that it was principally 

 during that period that the dispersion of the granitic blocks took 

 place. Now, if this period of submergence was a tranquil period, 

 what is become of the sedimentary beds which must necessarily have 

 been formed in a quiet sea ? Instead of such beds, of which there is 

 scarcely a trace, we have one part of the region swept almost perfectly 

 clean of all recent sedimentary matter, and another part occupied by 

 the tumultuary deposit of the Till. I cannot doubt but that a con- 

 siderable portion of this deposit was derived from the valleys of the 

 Highlands, and was transported from thence by currents which were 

 produced by repeated elevatory movements of that part of the district, 

 and which not only swept away any new sedimentary deposits, but 

 also still more deeply excavated the pre-existing valleys. I can 

 conceive no other agency of sufficient power to sweep out accumulated 

 matter from some of the Highland valleys (as that for instance of 

 Loch Awe), which are deep and extensive, but want all open com- 

 munication with the Lowlands or the ocean. These same currents, 

 partially at least, I consider to have been instrumental in the final 

 dispersion of the blocks previously brought down from the mountains, 



* Quart. Journ. Geol. Soc. vol. iv. p. 10, 1849. See also Sir R. Murchison's 

 account of Siliu-ian Conglomerates in Ayrshire, op. cit. vol. vii. p. 149 et seq., 1851. 



