A PERLITIC PITCHSTONE FROM TWEED RIVER. 309 



prismatic cavities whose lengths are parallel to the vertical axis, 

 and which appeared to be filled with glassy material. 



Crystals of magnetite similar to those included in the hypersthene 

 are scattered through the rock, but are most numerous around the 

 grains of hypersthene, while some are partially in the hypersthene 

 and partially in the glass. These facts seem to indicate strongly 

 that the magnetite was originally contained as inclusions in the 

 hypersthene and has been set free by partial solution of the latter. 



Though the phenocrysts are traversed by numerous cracks no 

 apparent dislocation of the pieces has taken place, and this, in 

 conjunction with the fact that many of the cracks pass out into 

 the surrounding glass affords strong evidence that this cracking 

 took place subsequently to the cessation of flow. 



The glassy matrix is transparent and colourless in thin sections 

 and thoroughly vitreous, but at the same time filled with innumer- 

 able microlites and crystallites, which under a one inch objective, 

 appear as minute twisted black lines. These by their varying 

 concentration serve to indicate the excellent lines of flow and 

 vortex motion in the magma. Under high magnifying powers 

 the minute bodies which are so densely distributed through the 

 glass may be divided into two classes, the proportion between 

 which varies in different parts {Plate 45, fig. 2). One of the classes 

 consists of plagioclase felspar microliths, and is most typically 

 represented by straight rod-like bodies terminated by crystalline 

 faces and traversed by one or more cracks which sometimes 

 appear parallel to one of the end faces (Plate 46, fig. 1). These 

 minute rods, which average about *02 mms. in length, are crystal- 

 line and react on polarized light. Their extinction angles, though 

 difficult to measure with any approach to accuracy, vary from 0° 

 to about 25° with the longer axis. They are doubtless lime-soda 

 felspars rich in lime and the elongation is probably parallel to the 

 axis ' a.' Less perfect forms occur having forked terminations, 

 while others resemble a dice box in shape. All the individuals of 

 this class lie with their longer axes parallel to the lines of flow. 



