Washington — Chamockite Series of Igneous Rocks. 331 



11 miles south of Madras, was examined. It is a black, medium- 

 grained rock, composed entirely of glistening grains, from l-3 mm 

 in diameter, of pyroxene and hornblende. The luster from the 

 cleavage faces is especially marked in the two largest parallel 

 faces of the specimens, indicating a schistose structure. The 

 microscope shows that the rock is composed essentially of 

 hypersthene and hornblende, with much less augite, a small 

 amount of magnetite and very little apatite. The texture is 

 granitic, all the constituents being anhedral, with consertal 

 fabric. The specific gravity of a similar specimen is given by 

 Holland as 3-333. 



The abundant hypersthene is colorless, with a pleochroism 

 which is similar to that in the preceding types, but much more 

 intense : c blue gray, a and b flesh pink. The crystals usually 

 carry the characteristic schiller inclusions, in the form of exces- 

 sively thin, straight rods, which commonly form a core in the 

 center. In practically all longitudinal sections the cleavage 

 cracks are rather coarse, and the extinction is frequently 

 oblique. This fact, which has been noted in the case of the 

 hypersthenes of the preceding types, at first gave rise to the 

 idea that the supposed orthorhombic pyroxene was in reality 

 monoclinic. Dr. H. E. Merwin, however, very kindly ex- 

 amined the mineral grains and found that the refractive indices 

 are much lower than that of augite, about that of hypersthenes, 

 that they are optically negative, and that in cleavage flakes the 

 extinction is always parallel to the cleavage cracks. This 

 pyroxene, therefore, is certainly hypersthene, and the appar- 

 ently anomalous oblique extinction is to be ascribed to the 

 non-development of the usual prismatic cleavage, the coarse 

 cleavage cracks seen being due to the parting or cleavage 

 parallel to 5(010). 



Augite is much less abundant, and closely resembles that in 

 the St. Thomas' Mount norite, except that the pleochroism is 

 a little more marked. Some of the augites also show schiller 

 inclusions. The abundant hornblende is of a rather bright 

 yellow green color, with a pleochroism almost identical with 

 that in the norite, except that all the tones are decidedly 

 greener. The hornblendes also carry the schiller inclusions, 

 which are coarser than in the hypersthene and sometimes 

 slightly curved. All the hornblende is primary. 



Small magnetite anhedra are fairly common, occurring both 

 between and as inclusions in the other minerals. The usual 

 small apatites are present. No olivine could be detected with 

 certainty, nor could I find any of the green spinel mentioned 

 by Holland. There is absolutely no feldspar of any kind. All 

 the constituents are perfectly fresh and unaltered. 



