88 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. 



number of well-marked quartzose grains is not great ; and the fel- 

 spathic grains are certainly somewhat altered ; and it is difficult, if 

 not impossible, to say how much they are changed, and to be sure 

 that some of the grains which look like felsite are not partially 

 decomposed felspar altered into a felsite-looking substance. More- 

 over, since it is only those quartz grains which have a somewhat 

 unusual structure that can now be recognized with perfect confi- 

 dence, they necessarily make it appear as if the original rock were 

 of more unusual character than it really was. I have done my 

 best to form a satisfactory conclusion; and allowance must be 

 made for the unavoidable difficulties. 



The most striking peculiarity of the grains of quartz is that they 

 often contain -many minute hair-like crystals, which possibly are 

 rutile, and also many minute granules and fluid-cavities, sometimes 

 with included crystals of alkaline chlorides. In my collection I 

 have no section of a granite or felsite which shows these cha- 

 racters combined. There are many of the hair-like crystals in the 

 quartz of the granite of Aberdeen ; but it is on the whole far more 

 transparent, and shows the milk-white appearance in far less 

 degree, than the quartz-sand of the schists, and also contains com- 

 paratively few fluid-cavities or granules. As far as the fluid-cavi- 

 ties are concerned, the quartz of the grains is far more like that of 

 the Cornish granites ; but this latter contains few if any of the 

 hair-like crystals, and many of the much stouter crystals of schorl. 

 I have also been able to detect in the grains in the schists a few 

 well-marked enclosures of what looks like a fine-grained felsitic 

 base, as though they had been derived from a quartz felsite; but 

 yet they are not sufficiently numerous to indicate exclusively an 

 extreme type of felsite. On the whole, taking into consideration 

 the character of both the felspathic and quartzose grains, the most 

 satisfactory conclusion appears to be that the material was, to a 

 considerable extent derived from a granite of a type very unlike 

 those of Cornwall, but in some respects analogous to that of Aber- 

 deen, though differing from it in being more like a quartz felsite. 

 Perhaps, then, speaking generally, we may say that it was mainly 

 derived from a felsite of medium character, or partly from a more 

 perfect granite, and partly from a felsite of more extreme type. 

 This conclusion, of course, applies only to the particular district 

 now under consideration. The source of the material, in the case 

 of other districts, could, I think, be ascertained in a similar manner, 

 at all events within certain hmits. 



Stratification- foliation. 



In those schists which have stratification-foliation, the large 

 flat crystals of mica lie in the plane of the beds of different mineral 

 nature ; but still the structure differs essentially from true strati- 

 fication. Sometimes the parallelism is of only a very general cha- 

 racter, and horizontal segregation has taken place to a large ex- 

 tent, in the manner explaiued when describing the specimens of 



