NO. 1173. ROCKS FROM UNITED STA TES-MKXICO BO UNDA R Y—LORD. 775 



Moil. No. 191 (No. 53272). Colorado liiver, right bank, Yuma, Arizona, 

 Mon, No. 20«; (No. 53270). Gila Mountains at (iilaCity, Yuma County, 

 Arizona, near Mon. No. 206 (No, 53271^. Eastern base Coast Range, 

 California, Mon. No. 224 (No. 53273). 



The specimens of granite vary but slightly in structure and miii(;ral 

 composition. 



These are, with one exception (No. 5327;{, original No. 513), coarse 

 grained, and show in some instances evidence of excessive mechanical 

 deformation (Nos. 53207, 532G0, 53270, original Nos.3()2,433, 45M) which 

 is revealed by foliated structure (No. 53270) or by a system of rifting 

 and the acconr[)anying formation of a secondary quartz-mosaic (Nos. 

 53207, 53209, original Nos. 392, 433). They are characterized mineralog- 

 ically by an extensive <levelopment of plagioclase, which occasionally 

 equals in amount the orthoclase. Microcline is quite common in those 

 specimens that have undergone mechanical (crushing. liiotite is espe- 

 cially i)lentiful in specimen No. 53268 (original Nos. 401, 405) from I'ozo 

 Verde and No. 53273 (original No. 514) from the eastern base of the 

 Coast Kange. The latter rock contains also an occasional green horn- 

 blende. (Quartz, orthoclase, and the accessory constituents muscovite, 

 magnetite, ai)atite, zircon, and titauite (Nos. 53269, 53273, original Nos. 

 433, 514) sire of normal development. 



The plagioclase may be regarded as oligoclase with an average extinc- 

 tion angle of about 5^ on i)inacoidal cleavage i)lates. The crystals show 

 the usual polysynthetic structure. In two sections (Nos. 53269, 53272, 

 original Nos. 433, 482) feldspar individuals were observed, composed 

 of an inner andesine core much deconqjosed, and extinguishing at an 

 angh^ of 9'^, and a perfectly fresh outer zone with the extinction angle 

 of oligoclase (5°). 



In many of the specimens of granite — notably Nos. 53267, 53208 (origi 

 nal Nos. 390, ;'>98, 405) — decomposition by atmosi)h(}iic agencies has 

 reached an advanced stage. The feldspar is alteie<l to kaolin and mus- 

 covite; the biotite to chlorite, epidote (No. 53208, original No. 395), and 

 calcite, and the magnetite to leucoxene (No. 53271). 



h. Oahhro-diorite. — Coast Kange^ California, Mon. No. 224 (No. 53274). 



This is a peculiar coarsegrained rock composed of quartz and pla- 

 gio(!lase, together with considerable biotite and diallage in about equal 

 proi»ortion. Accessory minerals are magnetite and ai)atite. 



The most interesting feature disclosed by the microscope is the para- 

 morphic alteration of diallage to green hornblende (uralite). This 

 process of molecular readjustment has taken i)la(;e without change in 

 bulk or appreciably alteriiig the contour of the original augite crystal. 

 The secondary amphibole has a distinctly massive structure in contrast 

 to the commonly fibrous character of uralite, but shows the usual crys- 

 tallographic orientation in regard to the jiarent mineral. On sections 

 approximately parallel the ])lane of symmetry (the angle c:c) measures 

 about 40*"^ for the augite and 18""^ for tiie inclosing hornblende. 



