108 David Forbes — Reply to Dr, T. Sterry Hunt. 



tary rocks." When I was in Canada, what little I saw of the Lau- 

 rentian rocks did not at all prove to me that they had been derived 

 from still older sedimentary rochs, but, on the contrary, whilst believ- 

 ing that the Laurentian gneiss, quartzites, &c., were of metamorphic 

 sedimentary origin, I inclined to the conclusion that the materials of 

 which they had been reconstructed had most probably been the debris 

 of eruptive igneous rocks, and this view I have maintained since 

 1854 with regard to some of the analagous Norwegian rocks which 

 Dr. Hunt claims to be Laurentian. To refresh my memory, how- 

 ever, I have read over the description of the mineral characters of the 

 Laurentian rocks in the Report of the Geological Survey of Canada, 

 pp. 22-49, but can find no evidence whatsoever to the contrary — and, 

 therefore, without disputing the correctness of Dr. Hunt's assertions 

 on points where he ought at least to be confident, I would ask whether 

 this statement is founded on facts or on hypothesis. 



Dr. Hunt devotes a whole page to what appears to be an inquiry, 

 as to who first showed that water played a part in igneous action, 

 a subject which may be of personal or historical interest, but which 

 is quite irrelevant to the questions under consideration ; for all 

 geologists will persist, notwithstanding whatever Dr. Hunt opines 

 to the contrary, in regarding igneous action as volcanic action and 

 volcanic action as igneous action, nor can they suppose for a moment 

 that any person, except one who never had seen a volcano in eruption, 

 could be blind to the evidence of his senses and deny the co-associa- 

 tion of vapours and gases with volcanic action ; — that the results of 

 Mr. Scropes' admirable researches should have been discredited 

 and ridiculed and declared unchemical, should be a warning in future 

 to chemists not to hazard such opinions without having studied the 

 subject in the field as well as in the laboratory. 



As Dr. Hunt brings forward the question of the density of quartz, 

 I may here state, what I omitted in my paper in the Chemical Neivs, 

 that all arguments based on this fact are completely invalidated by the 

 fact that the specific gravity of crystallised quartz out of true volcanic 

 lavas is 2-6, or the same as that of the quartz in granite ; and, further, 

 that Mr. Sorby's examination of the quartz out of these lavas com- 

 pletely proves that it was crystallized out of the melted rock, and not, 

 as Dr. Hunt would have us infer, merely entangled from the debris of 

 originally sedimentary strata. 



Having long occupied myself with the application of the micro- 

 scope in geology, and repeated most of Mr. Sorby's experiments 

 relating to this subject, I consider it superfluous to contradict 

 Dr. Hunt when he accuses me of not understanding Mr. Sorby's 

 views, being quite content with that gentleman having expressed 

 himself decidedly to the contrary. I would recommend Dr. Hunt 

 also to commence with the study of microscopic geology, and 

 can well imagine his being disconcerted when, on opening the last 

 number of the Geological Magazine, he found a few lines from Mr. 

 Sorby, quite sufficient to annihilate all the deductions he had so 

 elaborately arrived at from the study of that gentleman's memoirs, 

 with the object of making them serve his own purposes. 



