58 Forbes — On alleged Hydrothermal 



accept tHs definition, and cautions geologists against accepting such, 

 as a test of the eruptive origin of such rocks. 



The writer would state that his experience, both in the field 

 and out of it, confirms himin adhering to the old definition, and he 

 finds (in which he believes most geologists will concur with him) 

 no evidence in Mr. James Geikie's memoirs to shake his confidence 

 init.i 



The writer of these remarks does not, in this communication, 

 even wish to express any opinion as to whether the views adopted 

 by Mr. James Geikie are right or wrong ; but leaves it to the geo- 

 logical reader, after perusing them, to decide for himself how far 

 the conclusions arrived at by that gentleman are entitled to confi- 

 dence. He would, however, wish it to be understood that in 

 bringing forward these remarks, he is not in any way influenced by 

 any feeling of personality against a gentleman whom he has never 

 even seen; and he may state that the substance of this communication 

 was put upon paper long before the papers here referred to appeared, 

 having been intended as an answer to similar views'^ elsewhere put 

 forward, and based upon similar (in the writer's opinion) unsound 

 data. The appearance of Mr. James Geikie's memoirs, apparently 

 representing in some measure the views entertained by the Geological 

 Survey of Great Britain, decided the writer in at once protesting 

 against the nature of the evidence by which the opinions were 

 supported. No theoretical views can now expect to be accepted in 

 science without having had the evidence in their favour thoroughly 

 scrutinized and subjected to the cross-examination of the collateral 

 sciences, for times are much changed from what they were some 

 fifty years ago, when it was a common practice to explain effects, 

 apparently unaccountable, by referring them to the agency of 

 electricity, or some other then little understood cause, thereby 

 finding a convenient solution for avoiding the labour of working out 

 abstruse problems; it is, however, to be hoped that geologists 



^ The -writer takes tlie opportunity of here stating, that the results of a protracted 

 study of this subject (several of the details of which will be found in a series of 

 communications, which have appeared at intervals, since 1853, in the Magazinfor 

 Naturvidenskab, Skandinaviske Naturforskeres Forhandlinger, Philosophical Maga- 

 zine, Quarterly Journal of the Geological Society, Edinburgh Philosophical Journal, 

 and other periodicals), have induced him to conclude : — 



(1.) That when the geological epoch of the appearance of an eruptive or intrusive, 

 rock isknown, such rock will be found to differ essentially, in mineral constitution, 

 from similar rocks injected at a different geological period. 



(2.) That eruptive or intrusive rocks of identical mineral constitution, have made 

 their appearance or intrusion into the earth's crust at the same geological epochs. 



(3.) That the minerals, or classes of minerals, accompanying or associated 

 with such intrusive or eruptive rocks, may serve as a means of distinguishing 

 the several eruptions in geological chronology, in a manner analogous to the deter- 

 mination of sedimentary strata, by the fossils, or classes of fossils, which they may be 

 found to contain. 



' The views in question are far from being new, for under various dresses they ap- 

 peared, were discarded, and re-appeared from time to time, even from the infancy of 

 geology ; some of their advocates having even gone so far as to suppose that the state 

 of granite was one which all rocks would ultimately arrive at, after undergoing a pro-' 

 cess of slow but constant internal alteration or fermentation. . . ! 



