CARBONIFEROUS FORMATIONS. 27 
only about a tenth of an inch, where a single quartz grain fills up the width 
of the vein. (See fig. 4.) These veins become most prominent on the 
weathering of the rock, as the sand grains resist corrosion more than does 
the dolomite. Under the microscope the large grains in the filling are seen 
to be mostly quartz. These quartz fragments are of varying size, rounded 
or subangular in shape, and without any assortment or symmetrical arrange- 
ment. There are also many grains of feldspar, which is sometimes fairly 
fresh, and sometimes is altered to a muscoyitic aggregate. Angular frae- 
ments of dolomite identical with the wall rock are common, and vary in 
diameter up to an inch. These materials are inclosed in a cement of closely 
packed, minute, irregular grains of carbonate, which from its behavior with 
acids is probably dolomite. This dolomite, however, has not the structure of 
the crystalline dolomite of the wall rock, but is plainly fragmental in nature. 
The sandstone vems are not widespread, and were observed only in a 
few localities, and there are not sufficient facts to prove their origin. It is 
certain, however, that they fill fissures which were formed after the dolomite 
was consolidated into its present condition; and since this filling has become 
indurated into a hard and compact sandstone, its formation was probably 
not extremely recent. The well-rounded quartz grains show that they had 
been considerably worn by aqueous action previous to being laid down, and 
also that water was the vehicle through which they were introduced into 
their present positions. There is, however, no positive trace of stratification 
among the materials, nor any sorting such as is often observed in water-laid 
sediments, but grains of all sizes are confusedly intermingled and lie in 
every position, with no observable parallel arrangement of their longer 
axes. It seems, then, that the deposition was not slow, but immediately 
succeeded the formation of the fissures, and that the materials were all 
introduced at the same time. 
Mr. Diller' has described cases of sandstone dikes in California, which 
are developed on a remarkable scale in shale beds. These dikes present 
peculiar features which indicate that they are not sediments, but that the 
material was forced upward, mixed with water, from a lower horizon, and so 
filled joint fissures, and that the formation of these fissures and the injection 
of the sand were phases of earthquake action. Cases where such injections 
have actually been known to occur, as in the earthquake at Charleston, 
‘ 
1Bull. Geol. Soc. Am., Vol. I, p. 411. 
