74 ROCK-METAMORPHISM. 



adopt a twofold division, but to make the separation on a dif- 

 ferent horizon to that adopted by Sir William. 



If we have not misunderstood the proposed subdivisions, the 

 lowest one, which includes most of the Laurentian system of 

 Logan, is essentially composed of silacid rocks, only a thin zone 

 of crystalline limestone being included in it ; the next one 

 embraces, in addition to the silacid members, a massive zone of 

 hemithrenes (" crystalline limestones ") , in which are included 

 stratified dolerites, ophites, and other related rocks. Accord- 

 ing to Vennor, who had favourable opportunities for taking 

 measurements, the zone has " an average thickness of 5600 

 feet"* 



Mr. Selwyn proposes to make a system for the last group of 

 rocks, retaining for it Logan's name, Huronian f, but to add 

 thereto some other groups (certain members being slightly 

 altered). Respecting the correctness of this proposal there may 

 be some disagreement among American geologists. 



It has been remarked by Sir William E. Logan that "even 

 during the Laurentian period the same chemical and mecha- 

 nical processes which have ever since been at work disintegrating 

 and reconstructing the earth's crust were in operation as now. 

 In the conglomerates of the Huronian series there are enclosed 

 boulders derived from the Laurentian, which seem to show that 

 the parent rock was altered to its present crystalline condition 

 before the deposit of the newer formation, while interstratified 

 with the Laurentian limestones there are beds' of conglomerate, 

 the pebbles of which are themselves rolled fragments of still 

 older laminated sand- rock; and the formation of these beds 

 leads us still further into the past" {. 



Cordially agreeing with these remarks, we are prevented ac- 

 cepting Sterry Hunt's belief that in " pre-Cambrian times there 

 prevailed chemical activities dependent upon greater subter- 

 ranean temperature, different atmospheric conditions, and abun- 

 dance of thermal water, and that under these circumstances 



* Ibid. p. 264. This is Mr. Vennor's estimate for the " Hastings limestone 

 zone." His estimate for what is considered to be a corresponding zone in 

 Lanark township is not " less than from 5600 to 6000 feet thick " (Report, 

 1874-75, p. 143). 

 . t Canadian Naturalist, vol. ix. p. 30. 



J Quart, Journ. Geol. Soc, vol. xxi. pp. 46, 47. 



