SILURIAN BEDS. 9 
nite; and it is mteresting to note that those conditions were the same as 
those under which the mineral is formed at the present day. The continu- 
ation of the subsidence of the ocean floor is indicated by the gradual dis- 
appearance of the detrital material and the formation of the purer siliceous 
dolomite, which belongs to the Silurian age. There is, however, no dis- 
cernible break or cessation of deposition between the two periods, but all 
indications are that the sediments were deposited continuously. 
SILURIAN BEDS. 
Description —The pure dolomites above the sandy beds are generally light 
gray-blue in color, sometimes stained reddish; they weather yellow-brown, 
from the oxidation of the small amount of iron which they contain, and 
are hard and compact, with a fine frosty luster which is characteristic of 
these, as well as of the Carboniferous dolomites. This luster results from 
the structure of the rock, which is made up of small interlocking crystals 
of dolomite, nearly uniform in size. There are usually blotches, nodules, 
bands, and seams of chert, which is generally light gray in color. The 
nodules are often very irregular in shape; when they become elongated 
into seams or bands they generally conform to the bedding, although 
sometimes they cut across it, at various angles. The only noticeable 
difference between the bottom and the top of the formation is that at the 
top the dolomite is locally finer grained than at the bottom, the crystals 
often becoming so small that the frosty luster coming from their facets is 
very faint. 
Microscopic structure—In mineral composition this rock is like most dolo- 
mites, being made up of small, interlocking, nearly uniform crystals. 
Another mineral which is never absent from any thin section is quartz, 
generally pretty evenly disseminated in isolated grains of small but 
varying size. On casual examination they appear like detrital grains, but 
when carefully observed it is found that their outlines, instead of being 
rounded, or even regularly angular, are often irregular and sinuous, 
presenting reentrant angles and sudden bays, such as would not occur in a 
grain which had suffered any friction whatever. ‘The quartz, moreover, is 
clear and free from any break or crack, such as characterizes detrital 
quartz, especially in that derived from granitic rocks. The grains are 
usually smaller than the dolomite crystals, and the crystallization of the 
