268 FINLAY 



abundant as the white or colorless feldspars. Carlsbad twins 

 may occasionally be seen on these with the unaided eye. They 

 are dull and show something of the oily luster which is charac- 

 teristic of nephelite. Grains of titanite are scattered through 

 the rock, and minute pieces of pyrite appear from time to time. 



When examined with the microscope the rock is found to 

 consist of plagioclase, augite, biotite, titanite, magnetite, apatite, 

 pyrite and zircon, in the order of their relative abundance. 

 There are no porphyritic developments. The outlines of the 

 minerals (with the exception of apatite) are quite irregular, al- 

 though the feldspars are commonly rectangular. 



The plagioclase, as determined by the Michel-Levy methods 

 is in two kinds, labradorite with the composition Ab^, An^ and a 

 more acid andesine Ab^An^. Its substance is fresh and glassy, 

 though broken by many irregular cleavage cracks. The feld- 

 spars are twinned polysynthetically on the albite and pericline 

 laws, with very fine twinning lamellae. A zonal structure is 

 occasionally observed. 



The augite is in large irregular patches intergrown with bio- 

 tite. This variety of pyroxene has a high extinction angle. It is 

 light clear green and non-pleochroic. Twins may often be noted. 

 The enclosures are apatite and magnetite. (See PL XII, fig. 2.) 



The mica is a rich chestnut brown biotite, with strong ab- 

 sorption for rays perpendicular to the cleavage. Irregular 

 masses of nearly colorless titanite are from time to time noted 

 in the slide. Magnetite is abundant in large grains, now and 

 then surrounded by rings of titanite. Rarely it shows a ten- 

 dency to form rods growing out from the borders of the mica. 

 The apatite prisms are well developed. The results of two an- 

 alyses of this rock are given below, columns I and II. Num- 

 bers III and IV are placed beside it for reference. 



I. II. III. IV. 



SiOg 45-75 48.49 49.30 56.28 



AI2O3 18.51 18.99 22.46 14.23 



^'^2^^ 6.55 9.59 ^j2.04 4.69 



FeO 6.02 1. 00 ( 4.05 



MnO .16 



MgO 5.06 5.05 2.14 6.37 



