155 



IV. — The Results of Ice Action upon the Surface. 



Quantity of rock removed during the Glacial Period. — It has been 

 already incidentally mentioned that the total quantity of certain 

 kinds of rock traceable in the form of boulders in the glacial 

 deposits of the North of England is enormous, considering the 

 superficial area presented by the parent masses in sitvL. The 

 quantity of Shap Granite in the form of boulders removed by ice 

 from the parent rock has long been a subject of remark, seeing 

 that the superficial area of the rock in sitA does not much exceed a 

 square mile. Equally good illustrations of the point under consider- 

 ation are supplied by the boulders derived from the St. John's 

 Quartz Felsite, the Armboth Dyke, the Mardale Gabbro, and others 

 already mentioned above. The tough and durable nature of the 

 harder porphyrites and "ashes" of the Borradale volcanic series 

 has also, in their case, favoured their survival through a prolonged 

 course of rough treatment, and the boulders of these rocks conse- 

 quently occur in overwhelming proportions in the drifts of particular 

 areas in the north. These last mentioned rocks occupy a super- 

 ficial extent of many square miles ; but this has less to do with 

 their wide dispersal in the drift than has their extreme durability 

 under the action of mechanical erosion. This point is brought 

 into striking contrast by a comparison of the proportion these 

 boulders bear to those of Skidda Slate. In siiit both rocks occupy, 

 roughly speaking, about the same superficial extent. Moreover, 

 the glacial agent that affected the one, must have affected the 

 other to at least an equal extent. Yet in the drifts resulting from 

 this action, recognisable fragments of Skidda Slate, as a rule, are 

 to be found only in very small quantities j and are then generally 

 confined to the immediate neighbourhood of the parent rock. 

 Hardly any, or perhaps none at all, have survived the rough treat- 

 ment involved in a transportal to a distance of only a few miles. 

 On the other hand, boulders of the volcanic rocks from the same 

 area have been transported at least as far south as Cheshire, where 

 I have met with many well-known old friends from the north. 



