LITHOLOGY OF THE PORPHYRIES. 367 



their growth that they have produced a hypidiomorphic, granular mass 

 in which quartz grains occur abundantly. Occasionally the latter have 

 some of their crystallographic faces preserved. In acidity the rocks are 

 slightly above the average of similar masses from other parts of the world, 

 often reaching 77 per cent of silica (SiOg). 



LITHOLOGY OF THE PORPHYRIES. 



The fine grained acid rocks are generally of a reddish color ; there is, 

 however, a wide range from light pink to a dull, dark gray or purple. 

 In texture there is considerable variation. The groundmass is dense 

 and very fine grained. It assumes all gradations, from a coarse micro- 

 granite to a fine di vitrified glass. , Through the groundmass are scattered 

 abundantly large individuals of quartz and feldspar, the phenocrysts. 

 The fine grained, compact character of the rock enables it to resist degra- 

 dational influences in a remarkable way, as is shown by the fragments 

 loosened in jointing, which preserve sharply all their irregularities long 

 after the granite boulders have become perfectly rounded or entirely 

 decayed. The rugged topographic forms also clearly indicate the same 

 properties of withstanding weathering influences. The fracture of the 

 porphyry is splintery or subvitreous. 



What has already been said regarding the mineralogical composition 

 of the granite applies also to the porphyry. The chief constituents are 

 the feldspar and quartz, but the difference in the structure of the rock 

 results in very different relationships. The accessories, zircon, magnetite 

 and apatite, are found scattered through the groundmass. The ferro- 

 magnesian ingredients are not, as a usual thing, well developed. The 

 feldspars are the same as those of the granite and comprise the three 

 varieties, orthoclase, microcline and albite. They range from grains of 

 microscopic dimensions in the groundmass to large phenocrysts nearly 

 an inch in length. The porphyritic crystals of quartz are frequently 

 rounded, with the characteristic embayments due to partial remelting 

 before the original solidification of the mass. 



The most striking differences between the granite and porphyry are 

 shown when thin sections of the two rocks are examined under the micro- 

 scope. The fine grained matrix in which are imbedded the larger crystals 

 at once characterizes the latter type. The groundmass varies consider- 

 ably, from the typical microgranitic structure to an almost glassy one, 

 which, however, is usually so thoroughly devitrified that its original 

 character can hardly be recognized. The various phases which are es- 

 pecially noticeable are the microgranitic, granophyric, micropegmatitic, 

 felsophyric, vitrophyric and spherulitic. A particularly interesting 

 facies is the trachytic, in which the minute crystals of lath-shaped feld- 



