PARK RANGE. 139 



section under the microscope reveals, in its interior, a series of lines and 

 spots to which the milky appearance is due, a high magnifying power, 

 showing them to be an aggregation of liquid-inclusions, carrying some- 

 times water with a moving bubble, and sometimes liquid carbonic acid 

 containing a bubble which disappears with a slight elevation of temperature: 

 the same phenomena are seen in the quartz of the accompanying gneiss. 

 Associated with the gneisses of this region is a hornblendic rock, closely 

 resembling that from Rawlings Peak, but in which the granitic structure 

 predominates over the gneissic, and the hornblende shows a tendency to 

 distinct crystallization. It is a compact, medium-grained rock, composed 

 of hornblende, orthoclase, and plagioclase, with but a small amount of 

 quartz. Orthoclase appears to be the prevailing feldspar. The rock has 

 the mineral composition of a syenite, with much of the habit of an erupt- 

 ive body. Closely related to this rock is another made up almost exclu- 

 sively of feldspar and quartz, but carrying also some plates of white mica 

 in subordinate quantities. 



In the metamorphic series of Jack's Creek occur beds that in a marked 

 manner resemble intrusive bodies, and in the hand-specimen it seems impos- 

 sible to distinguish them from well-known eruptive diorites. A specimen 

 in the collection may be described as a compact crypto-crystalline mass, 

 with no observable regularity in the arrangement of the mineral constituents, 

 with a rough, angular fracture, and of a dark-gray color. In mineral com- 

 position, it appears to be cliiefly an admixture of dark-green hornblende 

 and white plagioclase, the latter frequently present in long, acicular crystals, 

 of a vitreous lustre. 



The western slopes are generally densely wooded, and the exposures 

 much less favorable for observation than on the eastern side. The predom- 

 inating beds seem to be the hornblendic rocks, and but one or two localities 

 require special mention. Under the lavas, at the head of Little Snake 

 River, is exposed a rock, which is different from any of those already men- 

 tioned. It is a compact rock, having no schistose structure, but a peculiar 

 banded appearance, owing to the parallel arrangement of the fine quartz and 

 minute mica flakes between the feldspar crystals. Both orthoclase and 

 plagioclase are present ; the former, however, is the more abundant, and 



