Reid.] 



The Igneous Rocks near Pajaro. 



177 



Microscopical Characters. — Under the microscope the rock 

 is seen to be composed essentially of light and dark green horn- 

 blende and a clear vitreous plagioclase, with a little more feldspar 

 than am pinhole. Both minerals are in smaller crystals than 

 appears in the hand specimen, each seeming crystal being com- 

 posed of several distinct smaller ones. The accessory minerals 

 are magnetite, apatite, biotite, and pyrite, in order of relative 

 abundance. The magnetite is in rather large amount, con- 

 spicuous under the microscope. Lastly, a little quartz occurs 

 inter stitially. The secondary minerals are chlorite, epidote, and 

 a little limonite. 



. The hornblende occurs in the usual prismatic form with the 

 faces (110) and (010) developed. The crystals average in length 

 3.97 mm. with a maximum cross section of .9 mm. Terminal 

 planes are entirely lacking. There are two varieties of the 

 minerals: a dark green, and a light green to nearly colorless. 

 Basal sections show besides the traces of prismatic cleavage, a 

 distinct parting parallel to (100). Longitudinal sections cut 

 parallel or nearly parallel to (010) show also a very distinct 

 parting on ( li o l) . This parting is developed best in the lighter 

 colored hornblende. The minimum angle observed between this 

 parting plane and the vertical crystallographies axis was 68° 54', 

 which would make h = I = 1 or the cleavage plane (101) . The 

 pleoehroism is C = bluish green ; b = olive green ; SL = yellow. The 

 absorbtion scheme is C > b > ft. The two varieties differ in 

 color, intensity of pleoehroism, and absorbtion, and the double 

 refraction is a little stronger in the light than in the dark variety. 

 The angle of extinction is the same for both, averaging 16°, and 

 the direction of extinction being nearly at right angles to the 

 transverse parting on (101). Twinning is frequent on (100), 

 the two main divisions of a twin often showing three or four 

 twinned lamellae between them. In sections cut parallel to (010) , 

 this twinning causes with the parting on (101) a faint "herring 

 bone" structure similar to that shown by augite. 



The two varieties of hornblende are crystallized in perfect 

 continuity with each other. Sometimes the dark mineral is the 

 center of a crystal; sometimes the light forms the neucleus. In 

 general it may be said, however, that the dark green variety is 



