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University of California . 



[Vol. 3. 



The free feldspar grains, which vary much in size, have the 

 form of laths or of angular fragments of larger crystals. In 

 character they are similar to those of the andesite fragments. 



The cementing material which completely fills the interstices, 

 is a zeolite with low double refraction and one perfect pinacoidal 

 cleavage. It is probably stilbite or heulandite, or perhaps both 

 are present. The specks of limonite, scattered through the rock, 

 are especially concentrated in the cement. 



The rock thus described is evidently an andesitic tuff, whose 

 materials to some extent were worked over and arranged by flow- 

 ing water. The character of the material, its homogeneity, and 

 the general angularity of the fragments, are evidence of pyro- 

 elastie origin. The lamination and the absence of fragile 

 vesicular fragments are the result of water action. 



Quarts Basalts of Cherry Creek. — Of the lavas overlying the 

 leaf-bearing tuffs of the Cherry Creek locality, a pyroxene-andesite 

 of the Hald's Canon type has already been noted. In this place 

 will be described a lava of somewhat peculiar character, which 

 may be designated as a quartz-bearing hypersthene-basalt. 



A typical specimen, viewed macroscopically, is seen to consist 

 mainly of a dead black, perfectly compact and very fine-grained 

 groundmass. In rather sparing quantity, but evenly distributed, 

 lie small crystals of greenish olivine and rounded grains of clear 

 colorless quartz. 



Under the microscope, the groundmass is resolved into a 

 typical "feltwork". The main constituent is pyroxene (both 

 rhombic and monoclinic), in slender prisms. Small feldspar 

 laths occur in subordinate amount, and there is a large quantity 

 of pale brown globulitic glass-base. Apatite occurs as an acces- 

 sory, while magnetite grains are rather abundant. 



The two varieties of pyroxene are not readily distinguished 

 except between crossed nicols. If in polarized light a prism is 

 placed parallel to one of the cross-hairs, it is oftenest found to 

 have its central portion in extinction, and bordered by two bright 

 lines, which extinguish in turn at a large angle. Most of the 

 crystals are thus found to be intergrowths of the two pyroxenes 

 after the plan that is usual in volcanic rocks, although separate 

 individuals of both varieties occur. More closely defined, they 



