COLOEADO ARCH^AN BODY. , 25 



Fork of the Cache la Poudre has, in general, a uniform lithological habit. 

 It is a coarse granular, easily-crumbling granite. The prevailing color of 

 the quartz is gray, while the feldspars, chiefly orthoclase, are in some local- 

 ities white, and in others deep red. The mica is almost always biotite, but 

 in varying proportions ; in some localities, it seems nearly wanting. This 

 region, as a whole, resembles, in many respects, the central granite of the 

 Laramie Hills. 



Northwest from the head of Sportsman's Creek, and south of the Lara- 

 mie Plains, is a considerable body of granite, which, in some particulars, 

 differs from the granite just described. It is more compact, more uniform 

 in composition, with a larger proportion of mica, without the tendency to 

 gneissic structure. The rock, in mass, is darker in color, and somewhat 

 resembles the granites from Sheep Mountain in the Medicine Bow Range. 

 Near the eastern base of the range, north of the Cache la Poudre, and east 

 of Park's Station, stands quite a prominent hill of massive granite, compact 

 in texture, and devoid of gneissic structure ; it bears a close resemblance 

 to the last-mentioned granite. 



Mica-schists and slates occur in narrow beds, similar to the one 

 already mentioned, interstratified in granites and gneistes : they occur 

 also in a more massive form in a number of localities along the foot-hills of 

 the range, and to the north, high up in the mountains. In the canon of the 

 Cache la Poudre, near the great bend, where it first cuts deeply into the 

 beds, the walls are built up with heavy beds of dark, close-grained mica- 

 schists. On Dale Creek, there are schistose, laminated beds, standing nearlj^ 

 vertical. An interesting locality for fhe slates is to be found near the en- 

 trance to the Big Thompson Canon, where they may be traced for a con- 

 siderable distance in a north and south line. They are a very fine-grained 

 admixture of quartz, feldspar, and mica, with a decidedly slaty cleavage. 

 Under the microscope may be detected fine flakes of muscovite, as in the 

 schist from Long's Peak, 



Narrow dikes of intrusive rocks penetrate the Archsean series of this 

 region. They appear, however, to be limited in extent, and to have caused 

 but little topographical and geological change in the great surface features 

 of the range. None were observed more than 10 or 15 feet in width; their 



