﻿316 G. M. Dawson — Rocks of British Columbia and Chile. 



Selwyn has compared in their lithological character to those of the 

 Huronian or Altered Quebec group of Eastern Canada, yet retain 

 ample evidence of their origin as volcanic sediments and igneous 

 flows, and hold some beds of crystalline limestone, and of argillite, 

 — the latter shoiving comparatively little sign of alteration. It is not, 

 however, intended at this time to enter into detail with regard to 

 these rocks, or of the yet more ancient-looking diorites and granites 

 of the Cascade Range, which are very possibly of the same age and 

 origin. 



Passing from rocks such as these, however, of which the source is 

 jet clearly demonstrable, to some of those of the Eastern border of 

 the Continent, one is led to think that sufficient prominence has not 

 been given, in endeavouring to account for their origin, to the possible 

 inclusion at dilferent periods of great m^asses of little weathered 

 volcanic products ; and that while in Britain the importance of such 

 material has been fully recognized, and it has been found to occur 

 at many stages in the geological scale, — forming in Cumberland from 

 12,000 to 15,000 feet of "green slates and porphyries," in Wales a 

 great thickness of similar hard and more or less crystalline rocks in 

 the Lower Silurian alone, — it has scarcely been allowed a foothold 

 in Eastern America except in instances so patent that to deny its 

 origin would be absurd. In discussing the possibility of the produc- 

 tion of " metamorphic " rocks from ordinary aqueous sediments not 

 chemically their equivalents, by pseudomorphism and replacement, 

 and the chemical formation of sediments by processes not active at 

 the present day, much ingenuity has been employed, while the place 

 of volcanos in supplying ready-made the material of crystalline 

 rocks has virtually in too many cases been ignored. This action, 

 according to strictly uniformitarian principles, must be supposed to 

 have been at least as important at former periods as at present, and 

 very lately the Challenger soundings have added largely to our idea 

 of its influence, Mr. Murraj^ having shown in connexion with them 

 that in point of fact all deposits in the depths of the Pacific not 

 organic are volcanic. 



The rocks of the Huronian are, where I have studied them on the 

 Lake of the Woods, I have no hesitation in affirming, in great part 

 of volcanic origin ; these beds, described originally by Dr. Bigsby 

 as " Greenstone Conglomerates," being undoubtedly of this character 

 and connected with others not so evidently volcanic by transitional 

 materials, the whole associated with some rocks which must have 

 approached ordinary argillites in composition and with quartzites.' 

 If correct in this instance, as I believe them to be, similar con- 

 clusions will apply to a great portion of the rocks of other localities 

 supposed to be of Huronian age, as a perusal of their description in 

 the " Geology of Canada " will render evident. In the felspathic 

 and gabbro-like rocks of the Upper Laurentian, we have a series so 

 completely the same in composition with certain abundant modern 

 volcanic rocks, that the attempt to account for its composition by 

 pseudomorphism, or by the theory of chemical precipitates unlike 



1 Geology and Resources, 49tli Parallel, 1875, p. 52. 



