84 



in the case of the plutonic rocks ; but the evidence for it in Arran we 

 cannot consider sufficient. There are many exceptions to the recti- 

 lineal course of dikes here as in other places ; some of the dikes con- 

 verge towards the Corrygills shore, and the largest runs a long way 

 parallel to it, while one at least re-appears far inland beyond the 

 place of the supposed focus. Besides, so far from " the whole of the 

 hills between Brodick and Lamlash being composed of trap," this rock 

 is, in point of fact, confined to a narrow and thin capping along the 

 highest ridge between the two bays ; and Mr. Napier has overlooked 

 the great outburst of porphyry at Dunfion, which has a manifest 

 relation to the felspathic dikes on the shore, as well as the numerous 

 masses of claystone intercalated amongst the sandstone strata along 

 the northern slopes west of Corrygills. 



On these grounds we cannot admit that this speculation has any 

 value ; the apparent radiation arises from the circumstance already 

 mentioned, that the vast majority of the dikes range between the 

 points of extreme magnetic declination east and west. Some other 

 suggestions, however, of this paper have great value ; those, namely, 

 of a chemical nature, referring to the different degrees of fusibility 

 and rates of cooling among trappean rocks, which, if carefully 

 followed up, and experimentally illustrated, might lead to the eluci- 

 dation of many points still obscure in the natural history of the 

 hypogene igneous formations. 



Glacial Phenomena. 



58. The various accounts which we possess of the geology of Arran, 

 and the separate memoirs upon it, were written before glacial action 

 had been recognized in the production of superficial phenomena ; 

 and we are not aware that any geologist has since turned his atten- 

 tion to the subject. Those phenomena, indeed, had been observed 

 of which the ice theory is now considered to offer the most satisfac- 

 tory explanation ; but the island has not hitherto been examined 

 with the view to discover the direct evidence of the action of ice, such 

 as striated and polished rocks, and "roches moutonnees." Visiting 

 Arran in 1855, after having spent two months of the previous 

 summer in a district abounding in unequivocal evidences of such 

 action, I felt the greater confidence in undertaking this task. The 

 result may now be briefly stated ; details being reserved for the 

 notices regarding particular tracts, which will be given farther on. 



The remarkable peak of Cior-Vor, whose altitude is about 2,500 feet, 

 has been already (Art. 43) pointed out as the geographical centre of 

 the northern group of mountains. From its base the four great 



