308 GEOLOGY OF THE DENVER BASIN. 



The groundmass is composed chiefly of plagioclase microlites with a 

 great deal <>f apparent orthoclase in irregular particles between them, and 

 sometimes forming a zone about the larger crystals. A positive identifica- 

 tion of the orthoclase is naturally impossible, but the chemical analysis 

 shows that its development is probable. Augite grains, magnetite, and 

 needles of apatite appear as in the rocks above described. Biotite is but 

 sparingly present. 



\'<> glass base now exists in the stream, unless in the contact zones. 

 But chlorite occurs in many angular spaces between microlites without 

 presenting evidence as to its origin. 



chemical composition. — The following aual\ sis, by .Air. L. (i. Eakins, is of very 

 fresh rock from the western body exposed on the southern slope of North 

 Table Mountain between the large gulches. 



Analysis of basalt from Table Mountain, Colorado. 



SiO* 49.69 



TiO a 85 



A1A. 18. 06 



IV.(); 2.64 



FeO 6.19 



MnO ... .13 



CaO 8.24 



MgO 5.73 



K o 3. 90 



Na.0 2.99 



lo, si 



CI 13 



llo 91 



1011.27 



A comparison of the analyses ot" the three basalts shows them to be 

 very similar, even to the abnormal ratio of the alkalis, and this fact is used 

 elsewhere as strong evidence that all belong to the same period of eruption. 



AUGITE-MICA-SYENITE. 



This rock occurs in small, irregular masses in the Archean near the 

 head of tin- north fork of Turkey Creek, Jefferson County. 



Description. — The rock is dark-brown iii color, quite compact on the 

 outer edges of the masses, while in the inner parts it is sometimes rather 



