8 GEOLOGY OF ASPEN MINING DISTRICT, COLORADO. 
are found to be of feldspar, mica, and occasional tourmaline; in shape they 
are generally subangular, sometimes angular, often rounded. he dolomite 
is like that of the pure dolomite beds above—uniform in grain and crys- 
tallized in gray interlocking rhombohedra, and often a small detrital grain 
of quartz or feldspar is entirely inclosed in a single crystal of dolomite. 
Thickness of beds— The thickness of the quartzite and sandy dolomite series 
varies greatly in the Aspen district, being greatest in the southern part, 
where it is about 350 or 400 feet, and gradually decreasing toward the 
north; so that in the northern part it averages 200 feet or less. 
age—No fossils were found in these beds, but they are lithologically 
identical and are continuous with the well-known series which extends over 
a large part of Colorado, lying around the borders of the granite area of the 
Sawatch. From fossils found in various places atthis horizon these. beds 
have been unanimously referred to the Upper Cambrian. It is not, however, 
clearly established at just what place in the beds the top of the Cambrian 
and the bottom of the Silurian should be put. Mr. Emmons’ has arbitrarily 
drawn this-line at the top of the shaly beds and the commencement of the 
more massive dolomite, and the same line is adopted in this report. . 
Conditions of deposition—According to Mr. C. D. Walcott,” Colorado, at the 
beginning of Cambrian time, formed part of a large island, which had a 
north-south extension of about 1,000 miles and a width of about 300 
miles. The island consisted of the Archean crystalline rocks and whatever 
ancient sediments had accumulated previous to Cambrian time. At about 
Middle Cambrian or the middle of Upper Cambrian time there was a subsi- 
dence of the land, so that a large part of it was brought beneath the waters 
of the ocean, and on this submerged area were deposited the sediments of 
the Upper Cambrian which have just been described. The first material 
was only slightly rearranged from the granite, which apparently was already 
disintegrated from atmospheric corrosion, while that laid down later was 
evidently deposited in water which grew continually deeper, as is shown 
by the careful sorting, by the small size to which the quartz grains became 
reduced, and by the mingling of dolomite with the detrital materials in the 
upper beds. When the.dolomitic materials became nearly equal in amount 
to the detrital grains, conditions were favorable to the formation of glauco- 
1§. F. Emmons, Mon. U.S. Geol. Survey, Vol. XII, 1886, p. 59. 
>Bull. U.S. Geol. Survey No. 81, 1891, p. 368. 
