306 GEOLOGY OF THE DENVER BASIN. 



abundantly through the feldspathic mass. This magnetite dust and the 

 clear apatite prisms are plainly later in formation than the larger grains 

 and prisms which are often included in the augite phenocrysts. 



The rock of the highest point on North Table Mountain represents 

 the inner part of the upper, thicker sheet. Its constitution is not very 

 different from that of the Ralston dike rock. By a kind of oxidizing 

 process to which it has been subjected all iron-bearing minerals — 

 magnetite, biotite, the alteration product of olivine, the small grains of 

 augite, and even the outer zone of the augite phenocrysts — have been 

 changed into an opaque, dark, reddish-brown substance (limonite), and this 

 gives the rock a reddish tinge in mass. 



At Castle Rock was found a dark nodule in the basalt, about \h 

 inches in diameter, which is an irregular aggregate of imperfect augite 

 prisms similar to the phenocrysts of the rock but of a decided green color, 

 with very little magnetite and plagioclase. This mass seems to be a segre- 

 gation of augite in the magma before eruption, and bears a significant 

 resemblance to the common olivine nodules in basalt, or to those of 

 amphibole in diorite or andesite. 



chemical composition. — The tvpical massive rock of the lower sheet was sub- 

 jected to analysis by Dr. AV. F. Hillebrand, with the following result: 



Analysis of basalt from Table Mountain, Colorado. 



SiO a - 51'. 59 



TiO> 84 



Ai,0 3 --- 17-91 



Fe.O 3 3. 81 



FeO - 5.18 



MnO Trace. 



CaO 7.24 



MgO 4.11 



K g O 3.83 



Na. ; 2.94 



P.O s 14 



CI 05 



H,0 1.24 



99.88 

 Sp. gr. 2.83 at 22i° C. 



This composition is so nearly that of the Valmont dolerite that little 



