CARBONIFEROUS FORMATIONS. 29 
in weathering makes it ordinarily easy to distinguish the limestone from 
the underlying dolomite in the field. The dolomite usually contains tiny 
chert areas o1 grains of silica throughout its whole mass; on weathering 
these project beyond the plane of the softer inclosing rock. This rough 
surface affords lodging tor red, yellow, and brown lichens, while the smooth 
surface of the limestone is usually quite clean. The iron of the dolomite, 
which oxidizes on weathering, also constitutes another distinguishing 
feature in the outcrop 
In the fresh rock the distinction is also easily made. The dolomite is 
made up of very small crystals of nearly uniform size, which give a frosty 
appearance to the rock; while in the blue lime the texture is varied, certain 
small areas being lusterless, while others show crystal faces much larger 
than those of the dolomite. In the mines the method of distinguishing the 
two rocks is to flash a candle on the specimen, when comparatively large 
glistening facets determine the limestone and many fine lustrous points the 
dolomite. 
Microscopic structure— Under the microscope the structure of the blue lime- 
stone is peculiar and uniform, except where it has been effaced by altering 
processes, such as dolomization, silicification, and ferration. The rock 
contains numerous tiny organic forms, chiefly Foraminifera, which are 
embedded in crystalline calcite. The tests of these organic bodies are of 
calcite, which differs radically from the coarse calcite in which they are 
embedded. Under low powers it appears quite amorphous; under high. 
powers it becomes a dark translucent mass, with many dimly polarizing 
specks, which, however, are not large enough to be positively recognized 
as individual crystals. In most of the material no polarization whatever 
can be made out. The interior of the shells, however, as disclosed by the 
sections, is filled with crystalline calcite like that of the cementing material. 
Crystalline calcite thus makes up about three-fourths of the rock, while the 
cryptocrystalline or amorphous lime carbonate makes up the remainder. 
The tests average about one-fiftieth of an inch in diameter. 
Dr. Rk. M. Bagg, of Johns Hopkins University, has kindly examined 
some thin sections of this limestone and finds the following types of 
Foraminitera: 
1. Endothyra sp. 4, Bigenerina sp. 
2. Nodosinella sp. 5. Valvulina sp. 
3. Textularia sp. 6. Lagena sp. 
