CRYSTALLIZATION OF SEPULCHRE MOUNTAIN ROCKS. 133 



still coarser-grained forms it becomes apparent that the cementing material 

 is quartz which has crystallized in irregularly shaped patches inclosing 

 many smaller feldspars. The size of these feldspars and of the interstices 

 between them is taken as the grain of the rock, and not the size of the 

 patches of quartz. For it is observed that as the rocks become more 

 coarsely crystalline the feldspars, which are plagioclase, increase steadily 

 in size and each quartz patch cements fewer of them, until in still coarser 

 grades the quartz forms allotriomorphic individuals between the plagio- 

 clases and does not surround any, so that in these varieties of rock the size 

 of grain is judged by the dimensions of the plagioclases and the interstices 

 of quartz. The patchy structure just described is that called micropoikilitic. 

 In the most siliceous varieties of the rocks the microstructure is differ- 

 ent. The smallest-grained forms appear to approach a granular structure, 

 in which, however, the feldspars exhibit a more or less rectangular shape 

 and the quartz shows a tendency to appear in minute, poorly defined 

 dihexahedrons. As the grain becomes larger the form of the quartz grains 

 becomes more pronounced (PI. XXII, fig. 2). They are rudely idiomorphic, 

 with sections that are in many cases equilateral rhombs, extinguishing the 

 light parallel to their diagonals. In the coarsest-grained forms of the dacites 

 these imperfectly idiomorphic quartzes are characteristic of the groundmass, 

 and reach a diameter of from 0.08 mm. to 0.10 mm. (PI. XXI, fig. 2). 

 Their surface is indented with the ends and corners of small plagioclases, 

 the structure of the groundmass being hypidiomorphic. These quartzes 

 often contain minute colorless inclusions in negative crystal cavities, which 

 have every appearance of being glass and correspond to the glass inclusions 

 in the quartz phenocrysts of the same rocks. The partially diomorphic 

 quartzes in the groundmass are to a slight degree porphyritical with espect 

 to the other constituents, but belong to the final consolidation of the magma. 



