Correspondence — Mr, David Forbes. 97 



pressed his surprise at Dr. Hunt being unaware of this, since he knew that the 

 principle had been long in use in a manufactory at Cork. 



Dr. Hunt has further applied this principle,^ and obtained very interesting results, 

 which he considered to be the counterparts of nature's operations ; and, remember- 

 ing that there are dolomite beds in the lower Silurian strata of Canada, at once asks 

 geologists to believe the rather hasty generalization that all the magnesian lime- 

 stones and gypseous beds were formed in a dense atmosphere of carbonic acid. 



Geologists, however, well knowing that the grand development of magnesian 

 limestones and gypseous strata occurred in periods when air-breathing animals 

 existed on the surface of the globe, could not believe that these animals actually 

 lived in a dense atmosphere of carbonic acid ; and had some of the more modern 

 great gypseous formations occurred in Canada, Dr. Hunt would probably not 

 have brought forward this theory. 



Vn. That quartz "can only be generated by aqueous agencies." 



Dr. Hunt, wisely no doubt, does not take any notice of my arguments against 

 this assertion, since they are facts, not opinions, and consist merely in pointing 

 out that volcanic lavas of Italy, Hungary, Peru, Bolivia, Chili, etc., contain 

 abundance of quartz often in well-defined crystals. In connection with this I may 

 here extract a passage from a letter received from Mr. Sorby, who writes, " I have 

 splendid cases of recent lavas with quartz, both in the shape of small crystals and 

 as rounded masses, like those seen in some older rocks ; and this quartz in both 

 cases (crystals and rounded masses) contains splendid glass cavities, just like those 

 in the felspars, the Arran pitchstone, and the various lavas ; thus we have complete 

 proof, according to my views, that quartz both can and has crystallized out from 

 a melted mass of rock." Now, in face of such facts, what importance, may I ask, 

 can be attached to such of Dr. Hunt's dogmatic assertions as, "that the composi- 

 tion of the primitive crust would have excluded free silica?" that quartz " is only 

 the result of a secondary process ? " etc. 



Vni. — " That granite is in every case a rock of sedimentary origin," 



Dr. Hunt makes this assertion in opposition to the opinion of many able men who 

 have well studied the subject. If he, however, only founds this opinion on the 

 presence of quartz in granite, the value to be attached to it may be inferred from 

 the remarks contained in the preceding paragraph. If he speaks as a geologist, 

 it may fairly be inquired whether he considers his Canadian experience sufficient 

 to enable him to arrive at such sweeping generalization. 



Sir Charles Lyell has stated that three things were essential to a geologist, 

 namely, "to travel, to travel, and to travel;" and such advice may be recom- 

 mended to Dr. Sterry Hunt before he ventures again to generalize for the world 

 on the strength of a local knowledge of a very minute part of the same. 



IX. — That volcanic rocks are merely ordinary sedimentary beds, melted by being 

 " depressed, so that they come within the action of the earth's central heat." 



In the Geological Magazine I ventured to inquire of " the author of this 

 ingenious theory by what mechanical arrangement he supposes strata, on the 

 surface of the earth, to be lowered down into a globe solid to the core ; " and 

 again, "how are we, according to this theory, to account for the fact that volcanic 

 rocks, taken from any quarter of the world, no matter how far distant from one 

 another — from Iceland or Terra del Fuego, from the Islands of the West Indies 

 or from those of Polynesia — that in all cases such rocks possess an absolute identity 

 in chemical and mineralogical composition, in physical and optical properties : 

 can any geologist be expected to believe that such rocks have been formecl by the 

 melting up of a mere mechanical aggregate of rock debris, possessing no analogy 

 whatsoever, and whose chemical composition, etc., is known to vary to the widest 

 imaginable extremes?" — questions as yet unanswered. 



Before concluding these remarks, I would here acknowledge that Dr. Hunt has 

 discovered an inaccuracy which occurs in my communication to the Geological 

 Magazine, where the position of steam in the imaginary original atmosphere is 

 by accident placed below that of air, although steam is in reality lighter — as a 

 moment's reflection would have shown. This error has not the most minute influ- 

 ence on any of my generalizations, as it is perfectly immaterial whether this 

 stratum be above or below that of air. 



I shall always be ready to admit at once any error which may be found in my 

 ^ Vide Chemical NewSy Sept. 13, 1867, p. 148. 

 VOL. V. NO. XLIV. 7 



