Dr. C. Callmvay — Notes on MetamorpMsm. 



223 



Squeezed quartzite in Glen Coul, 

 iSutheiiand. 



the quartzite in the Glen section. I have before me a piece of the 

 squeezed quartzite, taken from under the Caledonian gneiss, with 

 which it was in immediate contact ; and it will be seen from the 

 figure how powerful must have been the lateral thrust. 



In this sketch I have inserted only 

 the principal divisional lines, merely 

 to show the degree of contortion ; but 

 the spaces between these are, in the 

 specimen, filled in with numerous 

 finer lines, so as to display the schist- 

 like structure, to which I have re- 

 ferred. 



A three-fold change, we have seen, 

 has taken place in the quartzite. The 

 quartz grains have been flattened, they 

 have been rendered chalcedonic, and 

 sericite has been deposited between 



them. The flattening is easily explained by the pi-essure, but will the 

 same cause account for the other two effects ? Before I answer, I 

 would remark that in other localities, where the same conditions, 

 niini;s the pressure, have been reproduced, the metamorphism is 

 wanting. Masses of gneiss frequently rest immediately upon quartz- 

 ite ; and the felspar of the gneiss may well have supplied the 

 material of the sericite. However, little chemical change has taken 

 place. But excessive pressure appears to have induced not only 

 mechanical, but chemical, alteration. The modus operandi, it seems 

 to me, may have been the following. 



The probability that the gneiss, as well as the quartzite, contained 

 water will, I presume, be admitted. It is more than probable that 

 the enormous pressure generated heat. I do not call in the aid of 

 secular heat, for I do not suppose that in the Silurian epoch the 

 rocks in question were depressed to a sufficient depth. The heated 

 water would certainly be capable of extracting from the felspar of 

 the gneiss the material necessary for the pi'oduction of a hydrous 

 mica. How this solution was introduced into the quartzite, we can 

 but guess. Capillarity, aided by the heat ; osmose between the water 

 in the quartzite and the denser fluid in the gneiss ; pressure ; any 

 or all of these may have been concerned in causing the necessary 

 motion. Finally, it is not difficult to understand the deposition 

 of sericite between the particles of quartz, now flattened out into 

 continuous folia, or the conversion of the quartz into chalcedony. 

 The quantity of heat required would not, I think, be very great. 



The progressive alteration of a sedimentary rock towards its 

 junction with an adjacent metamorphic mass is thus seen to prove 

 nothing with reference to a passage between the two groups. 



In the Highlands, a false appearance of a gradation between two 

 series is sometimes produced by changes in the older rock at the 

 contact with the newer mass. In some places, the Hebridean gneiss, 

 thrown over on to the Ben More grit, which is often little more than 



