178 Correspondence. 



stand-point. The geological evidence in favour of metamorphism 

 is quite as deserving of study by the geologist, as the chemical evi- 

 dence is by the chemist. Mr. D. Forbes admits, indeed, that the 

 geologist has something to say in the matter ; but subsequently ob- 

 serves, in reference to certain geological evidence bearing upon the 

 origin of some granites, that chemical analyses " may not improbably 

 entirely anniliilate" it. If geological proofs and evidence are thus 

 liable to be " entirely annihilated,"' it is difficult to see why, in such 

 investigations, chemists and mineralogists should be bored with the 

 company of their hammer-bearing brethren.^ 



I have looked over my paper on the Carrick metamorphic rocks, 

 and must own that I have been careless and unguarded in the use 

 of chemical phraseology. Thus I admit that I have frequently 

 spoken of magnesia, of lime, and of alkaline matter, when I ought 

 to have stated that what I referred to was the magnesia of highly 

 magnesian minerals, and the lime of the carbonate of lime. I was 

 quite aware that the green tinge so characteristic of many of the 

 rocks within the area described was not due to the pi"esence of the 

 oxide of magnesium, but to that of certain minerals which contain a 

 large percentage of magnesia. Notwithstanding Mr. D. Forbes' 

 opinion, that "from the style of this memoir (but for its errors), it 

 might have been written by a chemist," I believe an impartial 

 reader will acknowledge that my arguments are based chiefly on 

 geological data, which in their very nature cannot be "annihilated" 

 by future chemical analysis. My references to the chemistry of the 

 subject are very meagre, as I had to content myself with the usual 

 tests employed by field-geologists, and certainly never dreamed that 

 any one should think that I based "my entire conclusions on 

 chemistry." 



Mr. D. Forbes imputes to me the opinion that " granite, diorite, 

 serpentine, porphyrite," etc., may be derived from one and the same 

 bed of greywacke. There is nothing in the memoirs to warrant 

 this ; but I have distinctly stated my belief, and have brought for- 

 ward evidence in support of it, that present differences of com- 

 position among metamorphio rocks point to original differences in 

 the composition of the strata. Certainly I am not alone in this belief, 

 nor can I agree with my critic that it is "a waste of time, thought, 

 and energy," to place such a view "before a rational public."^ The 

 greywackes familiar to Scottish geologists do not " consist essen- 

 tially of seventy-five per cent, of quartz," nor have they any definite 

 composition whatever. The term " greywacke," as used by Scottish 

 geologists, is applied exclusively to the hardened felspathic, and 

 sometimes argillaceous sandstones of the Silurian regions, in which, 

 although quartz is freqently present, it is yet by no means a neces- 

 sarily preponderating ingredient. They vary in texture from fine- 

 grained, almost compact, rocks, to pebbly conglomerates, 



1 Some interesting remarks on the value of chemical analyses of rocks will be 

 found in Cotta's "Rocks classified and described" (1866) p. 79. 



2 Similar opinions, based upon long-continued study of the chemistry of the subject, 

 ha-ve been placed " before a rational public," by, among others, Delesse, and Sterry 

 Hunt. 



