VARIATIONS OF TEXTURE IN IGNEOUS ROCKS 597 



are in general cemented by a mesostasis of coarse allotrio- 

 morphic quartz, which often includes or is intergrown with, in 

 poikilitic fashion, smaller crystals of feldspar, pale green horn- 

 blende, and sphene. The feldspar included in the quartz sinks 

 to very small dimensions, whereas the ordinary feldspar grain is 

 very large. The structure may be regarded as the coarsening 

 of the porphyritic structure, or at least closely related to it by 

 reason of the two generations. The large feldspar crystals are 

 partly orthoclase, but are chiefly finely striated. Optical deter- 

 minations of the striated crystals show microcline-anorthoclase 

 and albite. 



Granite, coarse (172 N.). — This rock is almost entirely like 

 175 N, and has a good deal of the peculiar structure of this 

 rock, but in general is more hypidiomorphic granular or truly 

 granitic. 



Analysis of the structures of the gra?tite-rhyolites. — The analysis 

 of the structure of the granites and rhyolites just described helps 

 toward a better understanding of their relation. 



164 N (a). Here are phenocrysts of all sizes, gradually 

 shrinking in size to the glassy (sometimes slightly devitrified) 

 groundmass ; i. e., the crystallization, instead of belonging to one 

 or two distinct generations, represents many generations, not 

 separable from one another. This is a proof of gradual and 

 equable hardening. It shows that the viscosity increased very 

 slowly and regularly to the point of final complete solidification, 

 the newer crystals having progressively smaller fields of crystal- 

 lization. 



165 N(a). This is like 164 N(a) except that the groundmass 

 diminishes, on account of multiplication of phenocrysts. This 

 marks a longer cooling period than 164 N(a), so long as almost 

 to permit of total crystallization as relatively large crystals. 



162 N(a). In this section the feldspar phenocrysts are not 

 connected by gradual transitions with the groundmass, which is 

 slightly coarser than that of 165 N(a) and is much more abun- 

 dant. We have, therefore, two distinct generations of crystalli- 

 zation, and this, together with the frequently broken character 



