776 PROCEEDINGS OF THE NATIONAL MUSEUM. 



This i)rocess of uralitization has progressed to such an extent that in 

 many instances the original pyroxene has entirely disappeared. 



The plagioclase of the gabbro-diorite is tabular formed after M (010), 

 but usually of irregular crystal outline and striated by polysynthetic 

 twinning lamella'. Sections parallel (010) show a maximum extinction 

 of 16°, which corresponds with that of basic andesine, 



c. Uralite-diubdse (proterobase). — Pozo Yerde, "ElBanorie," Sonora, 

 Mexico, Mon. ^"o. 141 (So. 53270). 



The specimens vary in color from grayish green (No. 53276, original 

 Nos. 401, 415) to a dark green (No. 414). They are characterized macro- 

 scopically by an outspoken even granular structure. Under the micro- 

 scoi)e a panidiomorphic development of the rock constituents is appar- 

 ent, although the original structure is considerably obscured by excessive 

 chloritization and uralitization. Miueralogically the rock consists of 

 green hornblende, lath-shaped labradorite, magnetite, quartz (No. 415), 

 and apatite, besides the secondary products chlorite, epidote, musco- 

 vite, and calcite. In slide {No. 415) the ferro magnesian constituents 

 are comijletely altered to chlorite and epidote. 



The hornblende of the proterobase has the physical properties of 

 uralite, and appears to be an alteration product from a preexisting 

 monoclinic pyroxene, retaining generally the crystal form of the parent 

 mineral (No. 414). 



The plagioclase crystals of this rock are extended in the direction of 

 a, and are usually twinned after the albite law. On M section a forms 

 an angle of about 20° with the principal sections of the nicols. Decom- 

 position products are muscovite, kaolin, and a colorless low double- 

 refracting mineral surrounding the altered labradorite crystals, and 

 resembling an outer zone of secondary enlargement. This mineral, 

 however, is uniaxial, 0]>tically positive, and extinguishes parallel with 

 the sides of the feldspar crystal (No. 414). It is insoluble in dilute 

 hydrochloric acid, and can be nothing else than quartz. 



il. Biorite-^porpliyry. — La Osa, Pima County, Arizona, Mon. No. 140 

 (No. 53275). 



The specimen represents a coarse grained iDorphyritic rock, contain- 

 ing abundant pheuocrysts of plagioclase, green ampbibole, biotite, 

 quartz, and magnetite. The groundmass reveals upon microscopic 

 investigation a similar mineral aggregation with a preponderance of 

 quartz and feldspar. 



The hornblende crystals are of prismatic development aud frequently 

 twinned after (100). They show strong lileochroism — dark green par- 

 allel 1"« and c, yellowish green parallel a — and an average extinction 

 angle on (100) of 17°. 



The plagioclase is tabular formed after M (010), and, apaft from a 

 well-defined zonal structure and a more perfect crystal outline, resem- 

 bles the feldspar of the gabbro-diorite very closely. The crystals are, 

 however, far from fresh; being extensively altered to muscovite and 

 kaolin. 



