Keith] TENNESSEE MARBLES. 367 
The marble differs from most of the rocks of the formation in 
being coarsely crystalline. It may have been altered after its forma- 
tion by the passage of water through the rock, dissolving and recrys- 
tallizing the carbonate of lime, or it may have been deposited in its 
present form. The shaly parts containing less lime are not crystal- 
line. The forms of the fossils inclosed in marble are plainly visible, 
although wholly recrystallized. The marble varies considerably in 
color, most of the rock, however, being of two types, a dark bluish 
gra}^ and a variegated reddish brown or chocolate. Of these two 
varieties the latter or reddish marble is considerably more common. 
Both are extensively quarried for ornamental stone. 
Workable beds are rarely over 50 feet thick, and usually in that 
thickness there is a combination of several varieties. Quarries far 
separated from one another have quite distinct series of beds, and 
each quarry has its special variety of marble. All marbles of this 
region are free from any siliceous impurity, and all of reasonable 
purity take a good polish and are unaffected by weather. 
The total thickness of the marble beds is by no means available for 
commercial use. The rock must be of desirable color, must quarry in 
blocks of large size free from cracks or impure layers, and must be of 
fine, close texture. 
The variations in all of these characters are duo to differences in 
the sediment at the time of its deposition. Carbonate of lime, iron 
oxide, and clay were deposited together with shells of large and small 
mollusks. The firmness of the rock depends upon a large proportion 
of the lime, while the dark, rich colors are due to the oxide of iron; 
but if the latter be present with clay in large proportion the rock 
becomes a worthless shale. The colors vary from cream, yellow, 
brown, chocolate, red, and pink to blue, in endless variety. Absence 
of iron oxide results in gray, grayish white, and white. The colors 
are either scattered uniformly through the rock or are collected into 
separate crystals or patches of crystals; forms such as fossils are 
usually of pure, white calcite. The curious and fantastic arrange- 
ment of the colors is one of the chief beauties of these marbles. Like 
the shaly matter, the iron oxide is an impurity, and the two are apt 
to accompany each other. The most prized rock, therefore, is a bal- 
ance between the pure and impure, and slight changes in the form of 
sediment result in deterioration or better quality. Such changes are 
common in most sediments and must be expected in quarrying the 
marble. Not only may a good bed become poor, but a poor bed may 
develop into good marble. 
Tests for absorption of water show a high resistance in the better 
grades of marble, and the rock is very well fitted for withstanding 
weather. Its crushing strength is also very high in the purer layers. 
Tests of a number of samples gave an average strength of 10,000 
pounds per square inch. 
