CAMBRIAN SEDIMENTS. 7 
by the other minerals. The detrital grains are surrounded by a cement 
of coarsely crystalline carbonate, probably dolomite. Associated with the 
granitic minerals referred to are many rounded bodies which are quite 
peculiar. These are made up in varying proportions of specular iron, 
dark-brown in color and with metallic luster; red, translucent hematite; 
limonite, yellow and earthy; cloudy siderite; quartz; matted actinolite; 
and calcite. These are generally confusedly and finely intergrown, 
although often they are in alternating zones. The cores of many of the 
grains are of siderite, which is oxidized around the edges to red hematite 
and limonite. Others have a core of specular iron, which appears to be 
very slightly magnetic; this alters, in part, to red micaceous hematite, but 
oftener to siderite. The structure of the grains suggests the alteration 
of glauconite, such as has been described by the writer from the Mesabi 
range in Minnesota.’ The iron is almost entirely confined to these spher- 
ical areas, and there appears to be no channel by which it could have 
filtered into the rock, nor any definite arrangement suggesting such an 
origin. The nature of the rock in which they occur, being. that of a 
sediment transitional between the zone of active deposition of eroded 
land materials and of the limestone deposition of the quieter seas, accords 
with this idea, for it is in such a transition zone that the peculiar conditions 
necessary for the formation of glauconite are obtained. 
Beds prominently glauconitic occur at this horizon throughout a large 
part of the Rocky Mountains. In Colorado they were noted in many 
places by Peale? at this horizon, among others on the Eagle River; and 
Mr. Eldridge*® has noted them in the Crested Butte district. 
Sandy dolomite —A hove this glauconitic grit there is a gradual and perfect 
transition to the massive siliceous dolomite of the Silurian. The transition 
beds are made up at the bottom of detrital material and of dolomite, with 
the former generally in excess; a little farther up the relative amounts of 
the two are about equal; and toward the top the detrital material is dis- 
placed by the dolomite. The rocks have the appearance of more or less 
ferruginous sandstones and shaly, siliceous limestones. In color they vary 
ereatly, being sometimes gray like the dolomite above, often reddish, yel- 
low, or brown. Under the microscope the detrital grains, besides quartz, 
1Bull. Geol. Nat. Hist. Survey Minnesota, No. X, 1894. 
2A. C. Peale, Annual Report of the Hayden Survey for 1874, p. 112. 
8G. H. Eldridge, Geologic Atlas U.S., folio 9, Anthracite-Crested Butte, Colorado, p. 6. 
