DESCRIPTIONS OF PLATES. 395 



PLATE in. 



1. APATITE FROM HOKNBLKNDE-MICA-ANDE8ITB. 



Thin section 37, magnified 240 diameters. Longitudinal section of an apatite crystal, showing 

 prismatic, pyramidal, and basal faces, and bearing glass with a gas bubble in a negative crystal 

 cavity, as well as a multitude of needle-like inclusions arranged parallel to the principal axis of 

 the crystal. 



2. AUGITE FROM PYROXENE-ANDESITK OF CLIFF HILLS. 



Thin section 102 a, magnified 55 diameters. Section showing a partial black border of magnitite, 

 wholly external to the augite crystal. 



3. AUGITE FHOM PYROXENE-ANDESITE OF RICHMOND MOUNTAIN. 



Thin section 77, magnified 55 diameters. Section showing a similar black border. 



4. APATITE FROM HORNBLENDE-MICA-ANDESITB. 



Thin section 37, magnified 240 diameters. Crystal of apatite with prismatic, pyramidal, and basal 

 faces, showing a basal cleavage and needle-like inclusions parallel to the principal axis of the 

 crystal. 



5. HORNBLENDE AND PARTLY ALTERED HYPERSTHENE FROM ANDES1T1C PEARLITE. 



Thin section 58, magnified 75 diameters. Section of a fragment of hornblende and hyperstheue in 

 juxtaposition, with magnetite and glass inclusions. The hornblende has remained fresh while the 

 hypersthene has been partly altered into fibrous actinolite. 



6. FELDSPAR FROM PYROXENE-ANDESITE. 



Thin section 102, magnified 25 diameters, iu polarized light with crossed nicols. Section of a triclinic 

 feldspar showing a marked zonal structure, the extinction angle at the center being 25 greater 

 than that in the outer zone. There are very narrow strips twinned after albite that do not show 

 zonal variation. At the center is an abundance of glassy inclusions that appear black between 

 crossed uicols. 



7. FELDSPAR FROM PINTO RHYOUTE. 



Thin section 122, magnified 31 diameters, with crossed nicols. Section of a triclinic feldspar, showing 

 by its outline that it is a Carlsbad twin, each half has also the multiple twinning of albite, and 

 each a different set of extinction angles. It is cracked transversely. 



8. APATITE FROM ANDESITIC PEARLITK. 



Thin section, 75 a, magnified 230 diameters. Longitudinal section of apatite, showing both ends ter- 

 minated by the base and pyramid and having well defined basal cleavage, with a small amount of 

 needle-like inclusions. 



9. HYPERSTHENE FROM ANDESITIC PEARIJTE. 



Thin section 58, magnified 40 diameters. Partly altered hypersthene with inclusions of magnetite, 

 glass, and small apatite crystals. It shows the fibrous decomposition taking place from the 

 extremities and fractures of the crystal and proceeding in a direction parallel to the principal 

 axis of the crystal. 



