DETAILED DESCRIPTION OF SLIDES. 99 



ing scarcely a trace of cleavage. Indications of zonal structure are visible; 

 i. e., the exterior layer of the mineral exhibits a somewhat different texture 

 from the remaining mass. The polarization of these hornblendes is remark- 

 ably brilliant, quite equalling that of ordinary augite. MaTiy of the crys- 

 tals are tvvirmed, one of them (14-23) being polysynthetic. A crystal of 

 considerable size is divided into halves of identical orientation by a narrow 

 layer of the mineral in a reversed position. 



In one part of the slide (13-27) are some minute scales of epidote 

 which appear to repi-esent the clinopinacoid, lifiiited by the base, the ortho- 

 pinacoid, and the j)0sitive hemidome. The direction of extinction is sensibly 

 perpendicular to the orthopinacoid. The same form of epidote is found in 

 other slides, e. g., in 371 at 17i-19. 



Slide 25li. Hierra Nevada, 1450. North drift 289 feet north. 



Partially decomposed dioritic porphyry. — Tliis is a grayish-grecu grauitic-looldng 

 rock, with brilliant hornblendes, and only a slight apparent tendency to 

 porphyritic structure. Under the microscope, however, it is seen to belong 

 among the porphyritic diorites. The feldspars are almost opaque, and it is 

 with some difficulty that they can be made out to be triclinic. The ground- 

 mass was evidently granular when fresh. There appears to have been a 

 little mica, now converted to chlorite and epidote. The hornblendes are 

 unusually interesting because present in all stages of decomposition. The 

 fresher ones are bright brown, without black borders, and solid except for 

 the well-marked cleavages. Other crystals seem to have undergone a spe- 

 cies of iibration in the direction of the cleavages. This fibration is accom- 

 panied by the presence of decomposition products, and each small elongated 

 cleavage prism seems coated with secondary minerals. Other hornblendes 

 are partially converted into chlorite, and a fine example is illustrated in Plate 

 II., Fig. 1 . Still others have passed completely into epidote. In some of 

 the partially decomposed hornblende crystals there are small crystals of 

 pyrite. 



Slide 194. McEihhcn Tunnel, 480 feet from eutrauce. 



Decomposed dioritic porphyry. — lu liaud spcclmcns thls roclc is grecuish-gray, 

 and somewhat porphyritic. Under the microscope it is seen to be greatly 



