278 FINLAY 



irregular and ragged pieces of augite are found associated with 

 such evidences of disturbance. The mineral is of a bright green 

 color with faint pleochroism to yellowish-green. The extinc- 

 tion is 40 degrees. 



Biotite, plagioclase and magnetite form the ground-mass, the 

 two first minerals being equally abundant. The feldspar is in 

 small lath-shaped crystals often arranged in rude flow lines. 

 They invariably show several minute twinning bands which give 

 extinctions indicating a more acid feldspar than labradorite. 

 They are believed to be andesine. They are automorphic and 

 have invariably crystallized before the dark minerals. 



The biotite never occurs in phenocrysts. It is always xeno- 

 morphic and interstitial between the feldspars. Small brown 

 rectangular pieces sometimes attain a length of . i mm. and from 

 that sink to microscopic dimensions. 



Magnetite, in grains and skeleton crystals, is scattered all 

 through the rock and titanite occurs in notable amounts. An 

 unresolvable glassy base makes up a small portion of the 

 ground-mass. 



The analysis and calculated standard mineral percentages of 

 this rock are as follows : 



Si02 -. 48.03 Orthoclase 11.68 



AI2O3 20.98 Albite 24.10 



FcgO.^ 7.06 Anorthite 36.70 



FeO 4.51 • Nephelite 1.99 



MgO 4.43 Diopside 8.37 



CaO 9.54 Olivine 6.25 



NagO 3.28 Magnetite 10.21 



K2O 1.99 H.p .61 



H.piio 21 99:91 



HjO^g" 40 



100.49 



G. I. Finlay, anal. The rock therefore belongs with order 5 

 of class II, in rang 4 and grad 3, hessose. 



C. Dike Rocks 

 Over fifty separate dikes were observed in the region im- 

 mediately about San Jose. They fall into two groups, the 



