PYROXENE ANDESITE. .Joi) 



iiitriuliii}? bays of gnmndiuass in crystals, witli iiicirc or It!ss roiuitleil angles, and tiiat 

 tlie outline of the border is generally that of the crystal, while that of the hornblende, 

 substance within is mostly irregular, suggests its being the result of a change in the 

 condition of the niolten magma when horul>lende ceased to crystallize out and pre- 

 viously formed hornblende crystals may have been partially melted, or rei)laced by 

 magnetite. This has in sonu? instances proceeded so far as to form pscudomorphs of 

 magnetite after hornblende (thin sections 90, 91, 87, .SS), as noticed by other observers. 

 Some oi' the pseudomorphs (thin sectiftu 91) show minute grains of augite uniformly 

 mingled with the magnetite,' suggesting more sti'ongly that there has been a melting 

 of the hornblende, followed by recrystallizatiou, utuler conditions which led to the 

 production of augite in place of the hornblende. This corresponds to the results 

 obtained in the artificial reproduction of hornblende, in which augite has always 

 been formed insteud of hornblemle. The hornblende is very free from inclusions, for 

 besides magiu'tite, only a small amount of colorless apatite is found, and in one or 

 two cases feldspar and augite. It is absolutely fresh iu all the sections made from 

 Richmond Mountain; as remarked before, it is not a constituent of the grouudmass. 



Magnetite is less abundant than the minerals just described, and of much less 

 importance in the composition of the rock, yet at the same time it is a constant ingre- 

 dient. It occurs in crystals and irregularly shaped grains, the largest about O-^'""" in 

 size, fi'om which they range to almost indistinguishable grains in the grouudmass; 

 it is very evenly disseminated, but not very abundant. 



Apatite is another constant factor, though of little importance; it occurs in com- 

 paratively large, stout crystals, 0-2'""' long by 0-0.~)"'"> broad, giving sharp hexagonal 

 cross sections and showing in longitudinal sections the pyramidal termination, P. It 

 is colorless, but in some instances is crowded with opaque microlites arranged i)arallel 

 to the vertical axis of the crystal. These give it a brown or gray dusted appearance 

 and exhibit an absorption parallel to the longest axis. One cross section shows these 

 microlites arranged parallel to the longest axis and in planes parallel to the prism 

 faces (section 79). The apatites also contain a few inclusions of glass with gas bubbles, 

 which are in negative crystal ca\-ities. The apatite is found closely associated with 

 the phenocrysts and seldom alone in the grouudmass. 



As acces.sory minerals biotite ranks first in importance, being of special interest 

 on account of its scarcity in this pyroxene-andesit( of Richmond Mountain and in the 

 similar pyroxene-andesite of Clitt" Hills, as compared with its great abundance in the 

 hornbh'ude-mica-andesite and andesitic pearlite of the district. It is found in only two 

 thin sections from Richmond Mountain (Xos. 79, 7.S), and in each of these there is (mly 

 a single individual of rounded form with intruding bays of grouudmass. The mineral 

 is brown, with strong absorption, and is filled with minute grains of magnetite deposited 



'The same observation liaa bi^en mndo by Ilr. K. OebbBke: Bcitrago zur Petrographio der Philippinon iiml der 

 Palan.Ins<'l. Noiics .Tahrbiidi fiir Min., etc., 18«1. li. I'.. I, p. 474. 



