AREAL GEOLOGY OF LOWER CLEAR CREEK 273 



the Troutdale Granite is rough and accentuated, forming in many cases deep canons with 

 steep walls and picturesque towers of rock. This area is in marked contrast with the 

 subdued areas to the north and south. 



Macroscopic. — In the hand specimen this granite appears as a hypidiomorphic mix- 

 ture of quartz, red or pink feldspar, and biotite. 



Microscopic. — Under the microscope we find that the quartz has a slight undulatory 

 extinction at times; the feldspar is chiefly an acid plagioclase with some orthoclase and 

 microcHne. The microcline frequently shows Carlsbad twinning in addition to the char- 

 acteristic cross-hatching. Sometimes the feldspar is seen in pegmatitic intergrowths with 

 the quartz. The feldspar is frequently found altered to sericite. In addition to these 

 minerals, we have muscovite, apatite, titanite, magnetite, and zircon. 



This is also an alkali granite. It differs from the Central City granite in the less 

 amount of femic or ferro-magnesian minerals, and in the presence of muscovite, wanting 

 in the Central City Granite. 



DiORITE 



The next lithologic unit of importance is represented by the numerous diorites, of 

 which one large mass east of Creswell is particularly interesting. This mass, extending 

 nearly to Clear Creek Canon, turns toward the southeast to a point north of Bear Creek. 

 Except in several local instances, this rock is a massive diorite, and shows little or no evi- 

 dence of dynamic metamorphism, though there is good'reason to believe, from the study 

 of the specimens collected and examined, that chemical metamorphism has been active. 



Besides this large mass of diorite, there are everywhere present in the Fundamental 

 Gneiss, and to some extent in the Clear Creek Gneiss, small dikes of diorite, for the most 

 part parallel to the schistosity or banding, and, while almost without exception greatly 

 sheared and squeezed by dynamic forces, are but rarely plicated. These, while not so 

 extensively metamorphosed as the inclosing rock, aVe still greatly affected by the same 

 or similar forces. The greater metamorphism of the smaller dikes seems to imply either 

 a greater age for these intrusions, as a rule, than for the larger mass of diorite, or else a 

 less ability to withstand the forces of metamorphism than was possessed by the larger 

 ma.ss of diorite. No attempt has been made to map these smaller dikes, only the most 

 important receiving attention. 



Macroscopic. — The rock consists of an even grained holo-crystalHne mixture of horn- 

 blende and plagioclase feldspar. In many specimens and in the larger masses of rock, 

 as a rule, there is no striking evidence of banded texture, but some specimens have a 

 banded appearance which at times becomes schistose. In some of the dikes there is a 

 marked coarse-grained texture, strongly suggestive of a typical gabbro, and here the com- 

 ponent minerals occur in large crystals with abundant development of garnet. 



Microscopic. — Under the microscope the rock is seen to consist essentially of a holo- 

 crystalline mixture of green hornblende and plagioclase, with some biotite, augite, quartz, 

 apatite, magnetite, titanite, epidote, garnet, and secondar\' calcite and actinolite; the 

 whole, as a rule, taking on a banded arrangement. 



The hornblende is of the common green variety, showing basal sections predomi- 

 nating, \vith the usual two prismatic cleavages. It is, as a rule, distinctly pleochroic, 

 but in some specimens shows only the faintest pleochroism. In the strongly pleochroic 



