MINERALIZED AND METHYLOSED ROCKS. 33 



from the 'Chemical and Geological Essays' (pp. 340, 341), 

 if an attempt should be made to set aside the crystalline cha- 

 racter of the rocks in question by teaching that it is not due to 

 metamorphism at all, in short, that these rocks are no more 

 than the ruins of adjacent pre-existing or Archaean masses, 

 produced by the mechanical degradation, and retaining, Avith 

 little or no alteration, the original mineral constituents of the 

 latter. 



But passing from European metamorphics, we beg to draw 

 attention for a moment to evidences which exist on Dr. Hunt's 

 side of the Atlantic. Referring to <( an array of facts," he declares 

 they "lead me to conclude that the whole of our crystalline 

 schists of eastern North America are not only pre-Silurian but 

 pre-Cambrian in age ",*. Hence they can only belong to " pre- 

 Cambrian times," when, as he has stated, " there are reasons for 

 believing there prevailed chemical activities dependent upon 

 greater subterranean temperatures, different atmospheric condi- 

 tions, and abundance of thermal waters"f, presumably exceeding 

 in energy those of later periods. Now, as the metamorphics of 

 Westchester and other neighbouring counties are a portion of the 

 crystallines in question, and lie within the t( eastern " region spe- 

 cified, we may safely leave it to Dr. Hunt to uphold his conclusion 

 against the evidences of late years brought forward by Professors 

 J. Hall, J. Dana, and others in support of their determination 

 that the rocks under notice, consisting of mica- and hornblende- 

 schists, hemithrenes, and ophites, similar to those characteristic 

 of true Laurentians, belong to the Upper Cambrian and Lower 

 Silurian periods J. 



It is obviously unnecessary to prolong these remarks ; for we 

 have only to appeal to the zone of chlorite and mica schists, 

 gneisses, quartzites, and subcrystalline limestones (Lower Silu- 

 rian by their fossils), stretching from Sleat in Skye to Loch 

 Eribol in Sutherlandshire, to make palpably erroneous the 

 doctrine that metamorphic rocks on a regional scale have only 

 been de veloped during pre-Cambrian periods. 



* Chein. and Geol. Essays, 2nd edit. p. 276. 

 t Address Brit. Assoc. Dublin, 1878. 



J Hall, American Journal of Science, &er. 3, vol. xii. p. 300 ; Dana, op. cit, 

 ser. 3, vols. xix. & xx. 



I) 



