780 PUOCEEDINGH OF THE NATIONAL MUSEUM. 



MacroHcoincally y)honocry8tH of j)laj:ji«)(;lii.s«;, liornblende, and biotito 

 art;, in most caKOS, easily i«!Cogni/abl<' in a dense ajjhanitic j^ronndiniiKS. 



Under the micioscope this {^roiindniass resolves itself into a finely 

 felted mass of delicate hornblende needles, plagioclase laths, and minute 

 grains of magnetite (No. .W288, original No. 127). 



The |)orj)hyiit)c plagioclase crystals are tabular formed after M (010) 

 and are almost rectangular in crosH sections, 'i'hey sliow (^liaracteristic; 

 albite twinning and many individuals are frequently united in ac(jord- 

 ance with the carlsbad law. Judging by an extinction angle on M (010) 

 sections of 15% they are aeid labradorites. Zonal structure is not 

 uncommon. The crystals consist of a corroded inner core of more basic 

 feldspar and an outer shell nX acid labradorite. The inner portion is 

 heavily charged with rock glass and extinguishes at a considerably 

 larger angle than the homogeneous outer zone (No. V21). 



The hornblende phenocrysts of the andesite are distinctly prismatic 

 in habit, showing the jirism fa/^es (110) combined with the<;lino])inacoid 

 (010). Twinning parallel (H)0) is (juite fref|uent. The crystals have a 

 brownish-green color and are strongly pleochroitic: a greenish yellow, 

 b yellowish brown, c greenish brown. The angle of extinction, meas- 

 ured on prismatic sections, is small : the angle c: c = about 12^^ 



This mineral is one of the first to und(;rgo decomposition. It is 

 replatjed in most sections by limonite and chlorite, or by epidote (No. 

 5:520:i, original No. 506). 



Biotite occurs in the form of yellowish-brown, hexagonal i)lates with 

 strong ])leochroi8m in the rock from the vicinity of Mon. No. 19 (Nos. 

 127, 142). It is, however, usually altered to magnetite (Nos. 5.' >2H0, 5.1201, 

 original Nos. 157, 467) or chlorite (No. .53201, original No. 464). 



Small, ])ur{)lish brown, j)rismatic crystals of apatite with strong 

 absorjition (E>0) are very x>lentiful in slide No. 127 from Mon. >o. 19. 



Magnetite is of normal development. 



The delicatie network of the andesite grou.'idmass is in many cases 

 greatly obscured by the infiltration of secondary quartz and limonite 

 (Nos. 5.'i288, 53292, .5.329:3, original Nos. 126, 492, .506), and calcite (Nos. 

 53289, .53201, .53293, original Nos. 157,464, 465, .508). 



Chalcedony occurs abundantly in rifts formed in the rock from Dog 

 Springs (No. .53290;. 



(J. Augite-andfHite. — riix inil>;.s west of Lake I'alomas, Mexico, Mon. 

 No. 20 (No. 53294). San Clermente Island, Pacific Ocean, Mon. No. 2.58 

 (No. .53295). 



The sy)ecimens of augite-andesite are dark brown in color, being con- 

 siilerably weathered. They differ from the rock just described in the 

 vitreous condition of the groundmass and by the presence of augite in 

 place of hornblende and mica. 



Augite occurs both in the groundmass and among the i)orphyritic 

 constituents. It is yjale yellowish green in color and has the large 

 extinction angle common to this mineral: the angle c:c = 47'^. The 



