Professor Geikie on the " Pitchstone" of Eshdale. 251 



abundantly dispersed through the vitreous base are well 

 striated triclinic felspars. Among them occur water-clear 

 unstriated Carlsbad twins which are probably sanidine. The 

 felspars seem to have been the first minerals to crystallise 

 out of the original magma. They are remarkably sharply 

 defined against the surrounding glass, and vary in size from 

 less than xoVo to fully -gV of an inch in diameter. They 

 frequently enclose portions of the glass, and drop-like 

 granules of augite. 



Though they commonly appear in the usual form of elon- 

 gated strips, with abrupt or indefinite terminations, they also 

 present examples of pyramidal endings, occasionally com- 

 pound where different groups of twin lamellae have their own 

 crystallographic termination. Here and there one or more 

 of these lamellae may be observed projecting in a pectinate 

 manner beyond the end of a strip-like prism. 



Tlie Augite occurs in well-defined forms. Seen with a low 

 power these appear as pale greenish yellow granules, some- 

 times dispersed singly through the glass, but more frequently 

 aggregated into groups. That the apparent granules have 

 really definite crystalline contours, however, may be seen 

 even with a weak magnifying power. More highly enlarged 

 they are found to present abundantly the characteristic mono- 

 clinic prisms and terminations of the volcanic variety of 

 augite. The smaller prisms project from the groups with 

 clear sharply defined forms, while the inner portions of the 

 groups have a fissured granulated character. The prisms 

 average perhaps about -g-J-g- of an inch in length. Very 

 perfect isolated prisms occur, which, when cut perpendicular 

 to the vertical axis, present the well-known eight-sided 

 sections. Besides the small abundant prisms, there are also 

 to be observed much larger and less perfect forms, having an 

 internal flawed structure as if they were built up of an 

 aggregation of granules, yet polarising uniformly and not 

 with reference to the individual granules. These larger 

 flawed crystals sometimes contain abundant and large glass 

 cavities. There are likewise to be observed large macroscopic 

 kernels or patches which to the unassisted eye appear like 

 fragments of compact basalt, caught up in the dyke. These 



