DE8CEIPTIONS OF PLATES. $95 



PLATE IIL 



1. APATITK I'ltOM lloliXIlLKXDK-MICA-ANIlESITE. 



Thill section 37, niagnitied 240 <Iiameters. Lougituilinal sntioii nf :iii ajiatitc crystal, showing 

 prismatic, pyramidal, and basal ftices, and boariufj glass with a gas Imblilc in a negative crystal 

 cavity, as well as a nmltitmlo of ni'i'dle-liUc in<-lusii)ns arranged jjaralbd to the princi|>al axis of 

 the crystal. 



'2. AUGrrE I'KOM PYKOXKNK-ANDKSITK OF CUFF HII.LS. 



Thin section 102 a, magnified 5.5 diameters. Section showing a ]),artial black border ofmagnitito, 

 wholly external to the augite crystal. 



:?. ArOITK FROM PVKOXENE-ANDESITF, OF RICHMOND MOUNTAIN. 



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



I. APATITE FROM HORNBLENDE-MICA-ANDESITE. 



Thin section 37, magnitiod 240 diameters. Crystal of .apatite with jirisniatic, pyramidal, an<l basal 

 faces, showing a basal cleavage and necdle-liUc inclusions parallel to the princip.al axis of the 

 crystal. 



5. liORXIiLENDE AND PARTLY AI.TEREW IIYPERSTHENE FROM ANDESITIC PEARLITE. 



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

 juxtaposition, with m.agnctite and glass inclusions. The hornblende lias ri^niained fresh while the 

 hypersthene has been partly alten^l into fibrous actinolite. 



6. FELDSPAR FROM I'YROXENF.-ANDESITE. 



Thin si'ction 102, magnified 25 diameters, in polarized light with crossed nicols. Section of a tricliuic 

 feldspar showing a nuirked 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 variatioiL At the center is an abundance of glassy inclusions that appear black between 

 crossed nicols. 



7. FELDSPAR FROM PINTO RIIYOLITE. 



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

 by its outline that it is a (^arlsbad twin, each half hius also the multiple twinning of albite, and 

 each a dilferent set of extinction .ingles. It is cracked transversely. 



8. APATITE FROM ANDESITIC PEARLITE. 



Thin section, 75rt, 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. 



;i. HYPERSTHENE FROM ANDE.SITIC PEARLITE. 



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

 glass, and small .ajiatite 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. 



