[GNEOUS FORMATIONS. 309 



coarse-grained. In the latter case brown biotite leaves and green augite 

 prisms are quite* easily distinguished l>v the naked <-v<-. The feldspar is of 

 ;i gray fjr bluish tint, and except where the cleavage faces show distinctly it 

 is scarcely recognizable. Biotite and augite seem abouf equal in quantity, 

 ;iik1 their predominance gives character to the mass, which bas macroscop- 

 ically a decided resemblance to sunn- minettes. In the compacter parts 

 of the mass the augite is much restrained in its development, while biotite 

 is still prominenl in minute flakes. 



Microscopical examination i>i' l»>tli coarse-grained and fine-grained 

 types shows them to possess the mineralogical composition shown by 

 many European "minettes" or "augite-syenites." The more important 

 constituents are orthoclase, augite, and biotite, with rhombic pyro 

 hornblende, plagioclase, quartz, apatite, and magnetite. The structure of 

 tin- coarser t \ pe is granular, while the more compad form show - something 

 nf an approach to porphyrinic structure through the prominence of the 

 pyroxene crystals. 



Orthoclase, the mosl abundant mineral, isdeveloped in irregular grains 

 which in the coarser rock are quite uniformly of a ]>;il<-, smoky-brown tint 

 owing to ;i cloud of extremely minute inclusions, the character <>(' which 

 could not be determined with a power of 1,200 diameters. They do not 

 seem to be fluid, however. These dust-like particles are often so evenly 

 distributed thai the turn- of the entire individual is uniform, while in other 

 cases the) seem to be arranged in more or less parallel rows or streams as 

 though on certain planes of growth; and certain portions, usualh the outer 

 borders of each grain, are clear. .Minute fluid inclusions ;m<l particles of 

 other minerals are common. < larlsbad twinning is frequent. Jn the coarser 

 rocks no plagioclase is visible, but in the finer-grained there are some small 

 stave-like crystals, of many laminae, which are probably oligocla 



Augite, the nexl constituent of importance, occurs in numerous crystals 

 and in small, round grains. Tin- crystals in the coarser type are I to 3 mm. 

 in length, and reach I mm. in thickness. In color they are pale-green and 

 perfectly nonpleochroic. They usually swarm with inclusions of magnetite, 

 biotite, :m<l especially apatite. There is also some glass. In form and in 

 abundance of mineral inclusions these augites resemble very closely the 



