106 UNIVERSITY OF COLORADO STUDIES 



in the form of sericite from the alteration of feldspar. Apatite included 

 in the feldspars, quartz, and biotite, is very common. Zircon in small 

 stout crystals included in quartz and all the feldspars is a constant 

 accessory. Hematite occurs associated with biotite from which it has 

 been derived, and is sometimes seen throughout a thin section in the 

 form of red powder. Magnetite occurs sparingly enclosed in hornblende, 

 but is a common secondary mineral. 



Basic Secretions. — Throughout the granite are numerous masses of 

 darker rock usually less than a foot in diameter, but sometimes three 

 feet or more across. They are commonly elliptical in outline, though 

 irregular and sharply angular forms are not rare. Common hornblende 

 makes up nearly one-half the rock. Plagioclase, biotite, and orthoclase 

 are the other essentials named in the order of importance. Small zircon 

 crystals, included in the feldspars, are numerous. The maximum 

 extinction angle noted in sections of plagioclase normal to the twinned 

 lamellae was 25 , somewhat higher than that of the plagioclase in the 

 biotite-granite. 



There is not much question that these "knots" are segregations of the 

 earlier-formed minerals rather than xenoliths. Although they are often 

 apparently sharply differentiated from the enclosing granite, in a few 

 instances they can be traced, by the gradual failure of hornblende and 

 the appearance and increase of quartz, into the normal granite. This 

 may be seen at the side of the railroad a short distance west of Salina. 

 It is probable that a few small masses, schistose in structure and badly 

 decomposed, are the remnants of rock older than the granite. 



Fine-grained Biotite-Granite. — A fine-grained gneissoid granite in 

 dike-like masses occurs at various points throughout the granite area, 

 but is best developed at Salina. A large body of this rock is exposed 

 west of the depot, and the same dike crosses the creek a few yards below. 

 Other exposures can be seen in the village on the north side of Gold 

 Run, and a similar rock occurs at Wall Street. Constituent minerals 

 are microcline in large amount, some orthoclase, quartz with undu- 

 latory extinction, and much biotite with basal sections in parallel planes. 

 The feldspars and quartz carry numerous zircon inclusions. 



The relation of this rock to the enclosing granite is not quite clear. 



