320 T. Mellaril Reade — Glacial Geology. 



contributed tbe larger store of rocky materials. The granite centres 

 on the submergence theory would, being at a greater elevation and 

 casting off glacial ice, continue supplying granite, boulders, and 

 pebbles for a much longer period than Anglesey. Thus, although 

 more distant in their derivation, the preponderance of granites on 

 Tryfaen is satisfactorily explained. 



I have already in various papers expressed my opinion that the 

 Low-level Boulder-clay and sands have been mainly formed iu 

 shallow waters when the subaerial denudation of the land con- 

 tributed largely to their formation. 



Their distribution is consistent with this view, hut not with the 

 view that the whole of their materials have been pushed along the 

 land from the northwards, and are mainly composed of ploughed-up 

 Irish-Sea bottom. It used to be a principle among glacialists that 

 the proof of any deposit being "ground moraine" was the prepon- 

 derance of local materials in it. Now the carrying powers of land- 

 ice have been theoretically extended to include almost every glacial 

 deposit in the category of ground-moraine, so that the boulders 

 forming the great bullc of the rocks in Boulder-clay hundreds of 

 miles away from their origin, and among which local rocks may be 

 searched for in vain, form no difficulty to the modern glacialist. 



But, say the suppoi"ters of the land-ice theory, the high-level 

 drifts are uncommon and sporadic, whereas on the submergence theory 

 they ought to be common and general. I confess myself unable 

 to see, if there be any truth in this argument, why it should not 

 equally apply to the deposits of an ice-sheet. The Drift has been 

 spread pi'etty evenly and generally over the low-lands, which is 

 consistent with glacio-marine deposition. It has been spread less 

 generally and more irregularly over the hill}'^ districts, but yet much, 

 more largely than the land-ice theorists would have us believe. It 

 must be an exceptional thing for shells to be present in these loose 

 deposits, yet nothing that has them not is admitted as marine-drift. 

 Again, on the low-lands numerous artificial excavations are con- 

 stantly showing us the nature of the subsoil, while on the hills we 

 have to wait for the occasional opening of a slate quarry or a gravel- 

 pit before we can ascertain what the superficial covering is composed 

 of or what its depth is. It was the accident that between 30,000 

 and 40,000 tons of sand and gravel were required for the Vyrnwy 

 filter-beds that discovered the existence of these extensive and 

 thick beds of sands and gravels with their remarkable molluscous 

 remains, previously unsuspected. I have little doubt that these 

 examples will multiply as further engineering works are carried out. 

 Quarries are not so likely to discover these beds, for quarrymen 

 avoid such places, as the expense involved in removing the " faj'," 

 as the overlying loose deposits ai'e called, lead their 02:>erations to 

 where there is less cover on the rocks. 



Another objection urged against the submergence theory is the 

 supposed impossibility of shells representing such diverse conditions 

 being congregated in one place. Sti'ange to say, Forbes was the first 

 to raise this difficulty with respect to the shells found by Trimmer on 

 Tryfaen. As a matter of fact, I have in my collection the whole of 



