220 Dr. C. Callaicay — Diorite and Acidic Gneiss. 



VI. — Can a Diorite become an Acidic Gneiss ? 

 By C. Callaway, D.Sc, M.A., F.G.S. 



I HAVE already answered this question in the affirmative in my 

 papers on " Metamorphism in the Malvern Hills," ^ though I 

 am aware that the assertion has been contested. Nevertheless, the 

 possibility that diorite and quartzite may have a common origin 

 was suggested by Prof. Zirkel as far back as 1876 ; and in 1892 

 Mr. W. S. Bay ley, of the Geological Survey of Minnesota, distinctly 

 affirmed that a certain quartzite was nothing more or less than 

 "a completely altered gabbro." My conclusion, therefore, is not 

 without support, and, though it has been strongly attacked, it refuses 

 to succumb. I trust I shall be allowed in the present paper to reply 

 to one or two of the objections to it. My remarks refer only to 

 the Malvern area. 



So far as I can gather, it has been objected that there is no true 

 passage between the diorite and the gneiss. I have, it is suggested, 

 made an error of observation, and have overlooked breaks in the 

 sections examined. No one has ever pointed out any of these breaks, 

 though I have exhibited series of specimens to the International 

 Geological Congress, to the British Association, and to the Geological 

 Society of London ; but I suppose that the belief in the extreme 

 improbability of the alleged change has led to the conclusion that 

 there must he error of oversight on my part. It is the old argument 

 of Hume over again — it is easier to believe that Callaway has gone 

 wrong than to believe that a quartzose gneiss should have been 

 formed out of a diorite. 



Many mistakes of this kind have no doubt been made. Before 

 the microscope was applied to the study of rocks, they were some- 

 times almost inevitable. The writer has studied some of them iu 

 Anglesey, Caernarvonshire, Donegal, Connemara, and elsewhere. 

 He is therefore fully alive to the risk of error, and has carefully 

 refrained from coming to the conclusion that one rock, passes into 

 another until it has been forced upon him by prolonged study in 

 the field and with the microscope. 



But perhaps I had better define what is the exact nature of this 

 passage from diorite to gneiss. In the first place, I do not mean 

 that there is somewhere a junction between them, at which they 

 are mixed up together in a very intimate manner. Thus, we often 

 find a granite and a grit almost in contact, with the granite passing 

 into an arkose, and the arkose into the grit. Sometimes observers 

 of undoubted competence have described this as a case of a true 

 gradation, and have inferred from it the metamorphic origin of the 

 granite, but no experienced petrologist would make such a mistake 

 now-a-days. 



In the next place, I do not intend simply to assert that there 

 is a true passage between the diorite and the gneiss. A mere 



1 Quart. Joum. Geol. Soc, August, 1889, pp. 485, 486 ; August, 1893, 

 pp. 418-420. 



