316 GEOLOGY OF THE DENVER BASIN. 



pebbles, the transition stages being those familiar to students of general 

 petrography. 



With the increase in augite conies greater basicity in general, and the 

 extreme is a rock closely allied to the basalts, though none distinctly 

 referable to the latter type was found. Apparently magnesia was not strong 

 in these lavas, for while augitic varieties are so numerous, hypersthene was 

 found in but one typical pyroxene-andesite from the summit of Green 

 Mountain. This also explains ,vhy basalts are not found. The basalt-flows 

 of Table Mountain are apparently contemporaneous with the Denver beds, 

 ami it is therefore probable that some basalt exists in the conglomerates. 

 No especially interesting mineralogical types were observed. 



structure. — All of the rocks are porphyritic, and possess the further 

 characteristics of lavas rather than of intrusive masses. Plagioclase and 

 augite phenocrvsts commonly contain abundant glass inclusions, and a 

 zonal optical structure is frequent in the former. Hornblende and biotite 

 are ordinarily much resorbed, and augite sometimes shows the same action. 



The groundmass naturally varies very much, but is usually to lie 

 described as niicrolitic, with fluidal structure in most cases. In the 

 hornblende- and mica-andesites the groundmass is often cryptocrystalline 

 or partly isotropic, with yellow or brownish globulites and patches of 

 tridymite. In the more basic rocks minute feldspar microlites are often 

 found in a dark base, usually obscured by hydration of the iron oxide. 

 None of the structures observed are such as are found, according to the 

 writer's experience, in intrusive rocks. 



Texture. — The great majority of the pebbles are naturally dense, but 

 in the coarser conglomerates there are vesicular pebbles in great variety, 

 representing nearly every observed massive rock type. In some of the 

 pores are zeolites. 



One rock, an augite-andesite, was found in many places in angular 

 fragments, from apparently a single horizon, and of all textures, from the 

 compact to the extremely vesicular. The porous modifications of this rock 

 contain henlandite and the new zeolitic species ptilolite. 1 The angularity 

 of the fragments and the various textures seem to indicate a local source 

 for this rock. 



'On ptilolite, a new mineral, l>y Whitman Cross and L. G. Eakins: Am. Jour. Sci., 3d series, 

 Vol, XXXII. 1886, pp. 117-121. 



