On the Metamorphic Origin of Certain Granites, 8^c. 287 



ON THE METAMORPHIC ORIGIN OF CERTAIN GRANITES, &c. 

 To the Editor of the GEOLoaiOAL Magazine. 



Deak Sir, — Will you kindly indulge me with space for a few- 

 words in reply to Mr. D. Forbes' letter in the last number of the 

 Magazine. 



The position which I have all along taken up, and from which I 

 do not in the smallest degree recede, is, that the field-geologist is 

 capable of affording valuable assistance towards the elucidation of 

 metamorphic phenomena. To show this was the main object of my 

 papers. My arguments were founded upon certain geological evi- 

 dence which, however objectionable and unsatisfactory to Mr. D. 

 Forbes, was, nevertheless, not of my own creating, but may be seen by 

 any one who shall take the trouble to examine in detail the regions 

 described by me. 



With regard to the petrological terms employed, I can only say 

 that I never made any pretensions to be a reformer of our nomen- 

 clature, and Mr. D. Forbes' rather warm invectives might therefore 

 have been spared. The looseness of our terminology is to be re- 

 gretted, but I have only used the terms in the sense in which they 

 have been for many years understood by British geologists. ^ 

 I shall have no cause to regret this correspondence however, if he who 

 knows so much about the subject, and who finds that even Phillips, 

 Lyell, and Dana are, or ought to be, ready to confess their errors, will 



^ Mr. D. Forbes takes me to task about my definition of greywacke as applied in 

 a general Avay to the great mass of the Silurian rocks of southern Scotland, and twits 

 me with the fact that Jameson and Macculloch understood by the term "greywacke" 

 a definite rock-species. But however definite an idea might attach to "greywacke" 

 some fifty or sixty years ago, that term ceased ere long to have any such precise 

 meaning, and came to be applied to the Avhole series of strata in our southern uplands, 

 formerly known as the "Transition -rocks." "There was a barbarous word," says 

 Mr. Jukes, (Manual p. 431,) "once in use as a kind of synonym of the term 'tran- 

 sition,' this was 'grauwacke,' a word now altogether discarded, even in a lithological 

 sense. It was one of those words that meant anything or nothing, and served merely 

 to conceal our ignorance of the true history of the rocks to which it was applied." 

 My critic " expected to have been referred to works specially devoted to the subject" 

 of petrology, but the reader will see that the terms complained of were purposely used 

 in the vague and general way in which they are commonly understood. It was, 

 therefore, quite unnecessary that I should make allusions to those authors whose names 

 Mr. D. Forbes so abundantly scatters through the pages of the Magazine. I may 

 just add, that my excuse for using the word greywacke at all, was the want of some 

 convenient general term which should not mean more than words like "Sand- 

 stone," "shale," etc. Some such term is necessary. Recurring for a moment to 

 another of my critic's complaints, the reader may be amused when he finds Mr. D. 

 Forbes admitting that, if instead of the word " certain," I had used "possible or even 

 probable," he would not have objected to the paragraph where I speak of the meta- 

 morphism of aqueous strata into crystalline rocks, like granite, diorite, hyperite, etc. 

 Now I have already quoted (p. 181) the statement of one of our most eminent author- 

 ities upon this subject, Dr. Sterry Hunt. That gentleman has remarked of certain 

 crystalline rocks that they " have by most geologists been regarded as rocks of igneous 

 origin, wherens they appear to be for the greater part imdoubtcdly altered sedimentary 

 layers or masses." Dr. Sterry Hunt, it must be supposed, is well acquainted with all 

 that has been done in this department of science, and since he thinks the use of the 

 word "undoubtedly" quite justifiable, I cannot see why the word "certain" should 

 require to be so loudly protested against. 



