Washington — CharnocJcite Series of Igneous Rocks. 329 



of small glistening black pyroxene and hornblende grains with 

 a little interstitial whitish feldspar. The specific gravity of 

 9.660 is 3-02 according to Holland. 



In thin section the two specimens are much alike. The 

 texture is granitic, as in the preceding types. There is much 

 soda-lime feldspar, which is highly twinned according to the 

 albite law, the extinctions indicating a general composition of 

 about AbjAn,, as was also observed by Holland. An unstriated 

 feldspar is present in almost equal quantity. This has a refrac- 

 tive index decidedly higher than the Canada balsam, and must 

 also be considered to be labradorite, at least in great part. This 

 conclusion is the more justified as the norm shows that very 

 little orthoclase can be present. Holland (p. 157) comes to 

 the same conclusion, on other grounds. 



In my specimens the pyroxene is mostly hypersthene, similar 

 to that already described, though the pleochroism is rather 

 weak. In the St. Thomas' Mount rock (but not in that of 

 Pallavaram) there is also a small amount of a grayish, slightly 

 pleochroic monoclinic pyroxene (augite), which is distinguish- 

 able from the orthorhombic by its very marked oblique extinc- 

 tion, higher birefringence and better cleavage development. 

 There is a considerable amount of hornblende, which, in the 

 St. Thomas' Mount rock is less than, and in that of Pallavaram 

 more than, that of the pyroxene. The color is a greenish 

 brown, with the pleochroism noted by Holland : a = pale brown- 

 ish yellow, b = yellowish brown, c = brownish green. The 

 hypersthene and hornblende would seem to be of about the 

 same age, as each is found included in the other. The horn- 

 blende is not secondary. A very few small rounded grains of 

 olivine are seen in both specimens. Small anhedra of mag- 

 netite are present, being more abundant in the St. Thomas' 

 Mount rock. There are some small apatites, but I could detect 

 no spinel. All the constituents are absolutely fresh. 



The mode of the St. Thomas' Mount rock is as follows, the 

 two pyroxenes being reckoned together : 



Feldspar 40-8 



Pyroxene 31 "0 



Hornblende 19'6 



Ores 8*6 



Analysis B is only partial, but the discrepancies between it 

 and A are similar to those in the case of the quartz diorite, 

 and are probably to be ascribed to the same causes. The 

 divergence in the two figures for Si0 2 and the presumably 

 lower alkalies are difficult to understand, unless it implies con- 

 siderable variation in the composition of the mass. The other 

 analyses will be discussed later. 



