PETROGRAPHY boo 



The pyroxene is rather commonly altered in part to uralitic amphibole, 

 serpentine, or chlorite, generally with more or less granular magnetite. 

 A corresponding partial alteration of the feldspars has given rise to fine, 

 scaly, apparently sericitic, aggregates and less commonly to kaolin. In 

 places epidote is also an abundant secondary constituent. 



TEXTURES 



The size of the mineral grains in the ruck has undoubtedly been deter- 

 mined in the main by the rate of cooling of the magma. Thus the coarser 

 rock occurs largely in the thicker portions, which retained their heat and 

 took much longer to crystallize than the thin sheets and dikes. In the 

 very coarse quartz-bearing facies, however, the degree of liquidity has also 

 been an important factor. These portions of the rock doubtless represent 

 the last remnants of the magma Avhich had become highly fluid, perhaps 

 almost a watery solution, by the concentration of the volatile constituents 

 from much of the adjacent magma that had crystallized earlier. The 

 dikes and thin sheets are even denser, as a rule, than the contacts of the 

 larger masses, and some of them are so homogeneous in appearance that 

 their igneous character is not readily detected, even with a hand lens, 

 although it may generally be inferred from the greater weight of the rock 

 in comparison with the dense black variety of shale. 



In the typical diabasic texture the angular spaces among the interlacing 

 lath-shaped plagioclase crystals are occupied by pyroxene. With increas- 

 ing proportions of the latter mineral it forms larger continuous areas in 

 which the feldspars are imbedded, producing the ophitic texture. These 

 textures are visible in hand specimens of the coarser varieties ; but in the 

 finer grained and dense varieties the aid of the microscope is required, and 

 the thinner dikes and sheets show all gradations to typical basalt with 

 glassy ground-mass. Scattering phenocrysts of pyroxene and, less com- 

 monly, of feldspar and olivine are developed in some of these denser 

 facies. In the varieties containing much quartz and orthoclase, whether, 

 in separate grains or in micrograph ic intergrowth, these minerals occupy 

 most of the angular spaces among the plagioclase feldspars and there is 

 much less pyroxene. With increasing abundance of quartz, orthoclase, 

 and the micrographic intergrowth of these minerals in approximately 

 eqnidimensional grains, there is a falling oil' of plagioclase and the tex- 

 ture becomes granitoid. 



ORDER OF CRYSTJ LL1ZATION 



Prom the diabasic texture of most of the rock, it is obvious that the 

 crystallization of the plagioclase feldspar was quite generally completed 



