130 UNIVERSITY OF COLORADO STUDIES 



there is no recognizable mineral. The quartz and feldspar phenocrysts 

 are rarely over 3 mm. in diameter; the mica crystals are usually smaller, 

 but occasionally have a diameter of 5 mm. Although the mica crystals 

 are almost black, thin cleavage plates are transparent and colorless. 



In thin sections the rock is seen to be almost entirely groundmass 

 which is microcrystalline and which may carry phenocrysts of one or 

 more of the above-named minerals. From some slides phenocrysts 

 are absent. Both orthoclase and plagioclase are present, the latter in 

 less amount than the former. The plagioclase has a maximum 

 extinction angle of 20 in the zone normal to the brachypinacoid. It 

 commonly shows alteration to sericite and calcite. Some orthoclase 

 phenocrysts show advanced alteration to muscovite, while others are com- 

 pletely replaced by kaolin. The brilliantly polarizing muscovite in 

 phenocrysts may possibly be primary, but in a rock so much altered it 

 is quite as likely to be a result of the leaching of biotite. No pheno- 

 crysts of quartz were present in the slides examined. 



The groundmass is an intimate mixture of poorly individualized grains 

 of quartz and feldspar in which are embedded lath-shaped feldspar micro- 

 lites and flakes of muscovite. The feldspar microlites are never more 

 than . 10 mm. long, and often are once twinned; rarely three lamellae are 

 present. They are frequently kaolinized or sericitized, and sometimes 

 calcite accompanies alteration. While muscovite may be in part a 

 primary constituent it is doubtless largely an alteration product of the 

 feldspars. In addition to its being intimately mixed with small grains of 

 feldspar, quartz occasionally occurs in irregular masses about . 2 mm. 

 in diameter. These grains bear no resemblance to phenocrysts and were 

 probably the latest product of crystallization, if not of secondary origin. 

 Irregular bands of minute brown specks seen under high powers doubt- 

 less represent the dendrites of oxide of manganese. No flow-structure 

 is apparent. 



AGE OF DIKE ROCKS 



There are no criteria by which the age of the dikes can be 

 determined, except within very wide limits. These rocks are considered 

 to be Tertiary eruptives, along with others of the Front Range which 



