MUSEUM OF COMPARATIVE ZOOLOGY. 1G9 



cither in single crystals or simple twins, which had the optical prop- 

 erties of orthoclase (although some might be triclinic) and enclosed 

 between them varying amounts of a decomposed base, and of quartz 

 which he regarded as secondary. The porphyritic crystals were deter- 

 mined as orthoclase (sanidin). The rock varied considerably in fresh- 

 ness in the several specimens. This rock, occurring thus as a surface 

 How, was called " Trachyte " under the classification used by Dr. 

 Wadsworth, corresponding in this case to the " Quartzless Porphyry " 

 of Rosen busch. 



During the season of 1889-90 eight sections of the keratophyre and 

 several sections of the detached anorthoclase crystals have been pre- 

 pared for microscopic study. Numerous crystals from the groundmass 

 have also been detached for the purpose of obtaining the specific gravity 

 and chemical analysis. IJiotite mica is often found in hand specimens, 

 and occasionally augite, although the latter has not as yet been detected 

 in any of the sections cut. 



In the light of our present knowledge and with further investigation 

 it is possible to supplement Dr. Wadsworth's accurate descriptions, and 

 to determine the feldspar pheuocrysts as anorthoclase, and the rock as a 

 keratophijre. The pheuocrysts occur as crystals elongated parallel to 

 a, with a sqiiai-e cross-section owing to the presence of the base and 

 brachypinacoid ; in addition to the two cleavages there is a rough trans- 

 verse fissuring. The crystals are quite glassy when fresh. In the rock 

 slides, in polarized light, the different feldspar sections show marked 

 optical peculiarities ; there is often a very fine single, or double (micro- 

 cline) twinning ; sometimes the whole of one section of the mineral 

 consists of irregular areas not extinguishing in common, which resemble 

 the phenomena produced by mechanical causes ; these areas contain 

 very fine lines crossing each other at various angles in the difierent 

 areas ; in other cases there is a very fine zonal structure. Sections pre- 

 pared parallel to the base show this tine irregular double twinning, and 

 give an extinction l'^ to 2° tjblique to the line of the second cleavage 

 (oo P co), and sections parallel to the latter cleavage give an extinction 

 about 9*^ oblique to the line of the first cleavage, with an obtuse posi- 

 tive bisectrix about perpendicular t(i the face, the acute bisectrix a 

 making the angle of 9° with the basal cleavage. These sections also 

 show sometimes a very fine indistinct microperthite striation. The 

 angle between the two cleavages was determined in the roilecting goni- 

 ometer as approximately 89° 42', about thpt of microline. The specific 



