PARTING QUARTZITE SERIBS. 15 
limestone or dolomite. There is also occasional tourmaline. The detrital 
grains are inclosed in a cement which is made up partly of finely granu- 
lated carbonate, apparently limestone or dolomite detritus, but mainly of 
a white opaque substance, which is probably kaolin. In places this matrix 
predominates in quantity over the included grains, 
In the typical case which has just been described there is no cementa- 
tion of the original grains by secondary silica, and therefore the rock is 
not a true quartzite, but a feldspathic and dolomitic sandstone. This sand- 
stone passes on the one hand into the sandy lithographic dolomite and on 
the other into a more siliceous variety which has a true quartzitic structure. 
This latter rock has a cement of secondary silica; further evidence of alter- 
ation from an original sandstone is the carbonate, which is concentrated 
into irregular crystalline patches. 
The third variety of the basal quartzite member—the sandy litho- 
graphic dolomite—is developed very gradually from the dolomitic sand- 
stone by an increase in amount of the dolomitic material and a decrease 
in the size and number of the detrital quartz grains. In sections of this 
rock the angular, subangular, or rounded grains of quartz are scattered 
in a mass of very finely crystalline dolomite, which is homogeneous and 
probably represents a lime mud. There are also many areas of crypto- 
crystalline dolomite; these have sometimes a rounded, sometimes an 
angular or irregular outline. This cryptocrystalline dolomite is the gray 
“lithographic limestone” of the field; in the hand specimen it appears as’ 
small irregular patches, which sometimes unite to form nodules or bands. 
The lithographic dolomite member— Where found in fresh condition, as in mine 
workings where oxidizing agents have not been very active, the prevailing 
color of the lithographic dolomite is a light, delicate gray, with a tinge of 
green. On oxidation, however, it turns chocolate-brown, and outcrops 
invariably have a brownish tinge. Owing to the fineness and uniformity 
of grain and the compactness of structure, weathering causes it to shell 
off on exposed surfaces, so that it presents in the outcrop rounded knobs 
of comparatively fresh rock, smooth to the touch. Microscopically the 
structure is that which has just been indicated. The main mass of the 
rock is of carbonate, cryptocrystalline or very finely phenocrystalline; 
through this are often disseminated small detrital quartz grains. In speci- 
mens which have undergone some slight alteration, apparently from under- 
