﻿Q. M. Dawson — Rocks of British Columbia and Chile. 317 



those of the present day, seems almost as unnecessary as it would 

 be to invoke a similar remote origin for the formation of an ordinary 

 sandstone. In suggesting volcanic agency as an important factor in 

 the history of the Lower Laurentian, more hesitation may be felt, 

 as the mere area covered by its rocks is so much greater than we 

 would expect to result from any system or linear series of volcanos as 

 at present known. It is still a fact that the greater part of the rocks 

 of that formation are just such as would be produced from the com- 

 plete metamorphism of volcanic products among which those of the 

 acidic class preponderated. Its limestones and iron ores do not oppose 

 the theory, as these, with the quartzites and graphites may have been 

 formed during periods of repose ; and it is also apparent that if con- 

 siderable areas of recently ejected volcanic matter were from time 

 to time exposed to sub-aerial influences, their decay would furnish 

 lime and iron readily and in great abundance to the surrounding 

 waters, there to be fixed by organic or other agency. 



Judging from lithological characters alone, and without presuming 

 to enter into questions of age, it would appear probable, or almost 

 certain, that volcanic sediments or other more or less immediate 

 volcanic products have assisted materially in the production of the 

 crystalline rocks of the Green and White Mountains, and their 

 probable southward continuations; the rocks of the Metamorphio 

 Quebec group, and those of the supposed Huronian of Eastern 

 Massachusetts and Maine. The concise description of these last 

 given in Dr. Hunt's "Chemical and Geological Essays,"^ might be 

 applied with scarcely a word of alteration to portions of the Meso- 

 zoic volcanic series of British Columbia. 



If we may be allowed thus to explain the building up of a great 

 thickness of the older rocks by volcanic action, we may economize 

 greatly in the call for geological time, which at present seems 

 desirable. The crystalline character of any series of rocks may 

 reasonably be supposed to depend more closely on their original 

 composition than on subsequent alteration, and in volcanic products 

 — which may be as finely stratified as any — we have the materials 

 of many of the rocks of the older crystalline formations. If, how- 

 ever, the action of volcanos in supplying matei-ials for rock-building 

 on a large scale be admitted as possible at any era in geological time 

 — and there is surely no reason why it should not be admitted — the 

 correlation of separated areas of crystalline rocks on lithological 

 characters alone, from the difficulty of completely eliminating 

 volcanic action, and the precise similarity of the volcanic rocks of 

 all periods, when they have sustained an equal degree of alteration, 

 becomes at least extremely hazardous. On the other hand, as already 

 stated, these rocks, due to the same period, may be found at a similar 

 stage of metamorphism and showing precisely similar characters for 

 great distances in certain lines of volcanic activity and disturbance, 

 and may also be accompanied by parallel belts of contemporaneous 

 materials of ordinary aqueous origin. 



1 p. 187, § 6. 



