and Igneous Rocks of Ensay. Ill 



recognisable as being part of that great series of slaty and 

 sandstone rocks, which I have spoken of under the general 

 term Silurian Argillites. But their inner structure differs 

 much from that of the normal type, and a principal distinc- 

 tion is that the argillaceous part has been converted almost 

 wholly, if not entirely, into minute flakes of mica. 



In some respects the less altered beds, and specially those 

 which are minutely " spotted," resemble some of the less 

 altered of the contact schists. 



As a rule, the quartz grains of these beds have been little, 

 if at all, affected, except in so far that in places they appear 

 to have been arranged with their longer diameters in line, 

 probably by pressure. 



The least quartzose and the most altered of these beds, 

 approach in their microscopical characters near to a mica 

 schist, in which the structure is very minute ; that is to say, 

 they retain the outward general appearance of the argillites, 

 but have been so far metamorphosed that there has been 

 produced in them the structure and composition of a mica 

 schist. In other words they are phyllites. 



The rocks which, in accordance with the distinction I have 

 now drawn, are to be considered as the true metamorphic 

 schists of Ensay, are found in three main varieties. The first 

 includes the quartz schists and the fine-grained mica schists; 

 the pinite schists form the second ; and the third includes 

 the gneiss. The somewhat peculiar rocks found at Content- 

 ment Hill connect the phyllites and the mica schists. 



The quartz schists always have either a magnesia-mica or 

 its alteration-product, chlorite. An alkali-mica almost invari- 

 ably occurs in connection with the small pinite masses and 

 veins in these schists. These constituents would bring such 

 a rock in its unaltered state within the term Mica-schist 

 rich in quartz. But as I have found in almost all cases, in 

 addition to the above-mentioned constituents, more or less 

 of a tri clinic felspar (albite or oligoclase), the schists might 

 be considered even to be a variety of a very siliceous gneiss. 

 Yet, as the micas are never absent, while the felspars are in 

 some cases wanting, I think the term Quartzose Mica-schist is 

 the most appropriate. 



The quartz of these schists is peculiar, and its study has 

 raised questions which it is not easy to answer satisfactorily. 

 The schists are metamorphosed sediments, and of all their 

 original constituents one might expect to find the quartz to 

 be least altered. I have observed that in the contact 



