and Igneous Bocks of Ensay. 



69 



is simple in structure, and has the appearance of the ortho- 

 clase found in granitic rocks.* These felspars of both 

 kinds are original clastic grains, and not regenerated by 

 metamorphic action. They are just such as I have fre- 

 quently observed in the Silurian sandstones of the district. 



The quartz grains are of different sizes, but as a rule they 

 have all their longer directions arranged one way, and linear 

 to each other. This arrangement I find in ordinary sand- 

 stones of the district, and is partly due to the process of 

 bedding; but in this case it has, I think, been increased by 

 pressure during the mechanical movements of the rocks. 

 Moreover, some grains have been broken across in directions 

 perpendicular to the foliation. The quartz grains vary both 

 as to the amount and nature of their inclusions. Some are 

 almost free from any, others have bands of fluid cavities, 

 and again others are full, not only of fluid cavities, but also 

 of microliths. The intersticial material representing that 

 which at one time was mud is now wholly converted into 

 mica, partly in scales, but also here and there in well-marked 

 flakes. This micaceous material forms foliations separating 

 the quartz grains. 



The quartz grains of these rocks are evidently of 

 clastic origin, but I have observed cases where secondary 

 quartz has been added to them, so that I could with difficulty 

 say where the original grain ceased. 



The quantitative analysis of this rock was as follows : — 



No. 



1.— 



-QuAKTZOSE PHYI 



-LITE. 



Si.0 2 



... • 



. 77-50 



A1.,0 3 .. 



, 



. . . . . 



. 1311 



Fe..0 3 . 



, 





1-62 



Ca.O 





... 



•82 



Mg.O . 







•98 



K o 





, . . 



232 



Na. 2 





... . 



2-64 



H 3 





; moisture 



2-08 





101-07 



Hygroscopic 



•50 



Sp. grav. 





.. 2-665 . 



* I use the word " granitic" merely as a convenient term of description, 

 implying no more than that the rock in question has the crystalline-granular 

 appearance of the granites, quartz-mica diorites, granitites, &c. 



