rYKOXENK-ANDESITE. 355 



bution in the West, though the rocks coutaiuiiig it can be shown to be of the same 

 character throughout. 



The largest individuals are characterized by a great abundance of glass inclu- 

 sions, which extend fioni the center outward, always leaving a border of feld- 

 spar free from inclusions. They are very inegular in outline and form a net-work so 

 thick in many instances as to equal in amount the f('l(lsi)ar which forms the meshes. 

 The glass is colorless and filled with ()pa(iu(^ grains and transparent globulites, besides 

 colorless microliters, whose high index of refraction and similarity to other more 

 determinable ones in the groundmass suggest their pyroxenic nature. There also 

 occur inclusions of the groundmass developed to the same degree as that surrounding 

 the feldspar crystal. The smaller individuals are freer from inclusions, but contain a 

 greater variety, the glass ones having sharp outlines, either round or nearly rectan- 

 gular, with a comparatively large gas-bubble and fewer microlitic secretions; li(|uid 

 inclusions are less frequent, with a briskly moving bubble, besides needles and stouter 

 prisms of a])atite, magnetite grains and rarely augite. The feldspar substance is 

 entirely fresh, without the slightest trace of decomposition; in some instances it is 

 intersected by cracks, in which hydrous oxide of iron has been deposited, and which 

 have led to the devitrification of part of the included glass, converting it into a yellow 

 cryptocrystalline aggregate. One single indi\idual contained calcite deposited along 

 lines of fracture. There is also present among the phenocrysts feldspars with quite 

 perfect cleavage, splinters of which i)arallel to the base give an angle of extinction of 

 0° and are j>robably oligoclase, their separation from aiiorthite by optical methods is 

 not possible in the tlun section. 



The microscopic lath-shaped feldspar crystals of the groundmass, averaging 

 0'03"'"' in length by 0-()().'i'"" in breadth are slender ])risms elongated in the direction 

 of the brachydiagonal, irregularly terminating in two or more needles of different 

 lengths and are in every case twinned with two or three lamelhe. The angle of 

 extinction measured from the direction of their length varies fi-om 0° to 26° and cor- ' 

 responds to labradorite or a less basic feldspar. Small square sections, not very 

 abundant, prove bj^ their diagonal extinction to belong to plagiodase. 



The second most essential component is pyroxene, which occurs in macroscopic 

 crystals averaging 1™" in length, a few reaching 2'"'" from which they diminish in 

 size to -OS""", having sharjjly defined outlines, well develo])ed faces in the prism zone, 

 of which the pinacoidal are much the larger, and occasionally showing the pyramid 

 P and rarely the base OP. The larger number of individuals, however, are not 

 crystallographically outlined, but appear as imperfectly develoi^ed crystals in more or 

 less rounded forms. It is without the black border that surrounds the hornblende, 

 but lias a narrow granular margin of i)ale yellow transparent grains, without tloubt 

 augite of final erystallization, Ibrmed at tlie time of solidification ol' the groundmass 

 about the primary larger individuals and to a lesser degree around the black bordered 

 hornblendes and magnetite grains, but in no instance about the feldsjiars. Its 



