356 Scientific Proceedings, Royal Dublin Society. 



has very kindly sent me the following notes on the mineral charac- 

 ters of the slate. By their insertion in this place my concluding 

 observations will be made intelligible. 



Bray-Head Slate. 



" The main mass of the slate appears to be composed of quartz, 

 sericite, and chlorite. 



" The quartz and sericite are so intimately intermixed, and the 

 individual constituents are so minute, that it is often extremely 

 difficult to make out the boundaries of the crystalline elements in 

 consequence of overlapping. The sericite scales are, however, easily 

 recognisable by their more intense action on polarised light. They 

 give colours when the quartz only polarises in neutral tints. 

 When viewed in ordinary light, the sericite appears colourless. 

 The scales show a tendency to an arrangement of their fiat faces 

 parallel with the planes of schistosity, as may be seen by rotating 

 a section at right angles to the schistosity under crossed nicols. 

 The greatest effect is produced when the planes of schistosity cut 

 the cross-wires at an angle of 45°. The parallel arrangement of 

 the sericite scales is, however, by no means rigidly observed. 



" The quartz, so far as it can be examined, gives no evidence of 

 a clastic character. 



" The banding in the slate appears to be mainly determined by 

 a variation in the amount of chlorite present. Some bands are 

 very rich in chlorite, other bands contain only a very small 

 amount of this mineral. 



" The chlorite occurs in somewhat irregular scales and scaly 

 aggregates. Sections at right angles to the easy cleavage are 

 markedly dichroic (rays parallel to the cleavage cracks very pale 

 yellow or brown, sometimes almost colourless; rays at right angles 

 to the cracks rich bluish- green). 



" In addition to the quartz, sericite, and chlorite, there are nu- 

 merous minute spots which appear nearly opaque by transmitted,. 

 and white by reflected light. With a magnifying power of 500, 

 these may be resolved into aggregates of more or less trans- 

 parent grains, which resemble the common alteration product 

 (leucoxene) after ilmenite. Sometimes these aggregates are seen 

 in association with black grains, and it seems probable, therefore,. 



