36 TRANSACTIONS OF THE [oer. 17, 
from the size of nearly 3 mm. down to fine dust. Quartz, in 
irregular grains, limpid, colorless, and apparently homogeneous, 
often 3 mm. in length; when examined in polarized light, these 
are found to separate into many small granules. Besides these, 
there may be sometimes observed a dark-green, transparent 
mineral, perhaps a form of amphibole, in a few minute particles, 
sometimes gathered in groups, often closely associated with the 
scales of phlogopite. Rutile, in afew very short and dark needles, 
which are opaque. Iron-ochre, in rare brownish particles, some- 
times seen in interstices of the other grains. Carbon (perhaps 
as graphite), in exceedingly minute, black particles or fine dust, 
scattered through the grains of dolomite in small quantity. 
Instead of being homogeneous, as the mass appears in reflected 
light to the eye or under a glass, the thin section reveals its 
grains sharply divided by distinct lines of demarcation. No 
finer material of any kind could be distinguished between the 
grains, as indeed no definable interstices occur. 
The practical conclusions from a study of the thin sections 
are as follows: 
1. The stone does not consist of larger grains bound together 
by a visible cement. All are of pretty uniform size, and the 
secondary cementing process, which has been indicated by the 
hardening of the stone on seasoning, must be brought about by 
the deposit of films of some soluble material, probably calcium 
carbonate, introduced throughout this fine network of division- 
planes, and so binding all the grains into a solid mass. 
2. The blue-gray color of the rock seems to be chiefly pro- 
duced by the dark smoky color of a certain proportion of the 
dolomite grains; and these in their turn appear to owe their 
color to the distribution throughout their material of a small 
proportion of fine particles of carbon and of pyrite dust. J am 
inclined to attribute the bluish shade of this gray color to a 
play of light from the brilliant faces of the minute crystals of 
rite. 
a The quartz grains were not an original constituent of the 
dolomyte. They possess the structure of a chemical deposit, 
and fill up cavities and interstices which existed in the rock 
before the period of folding. During the latter process, it is 
evident, heated aqueous solutions, carrying silica, permeated 
the somewhat cellular stone, caused the formation of a little 
magnesian silicate (phlogopite), and filled up the cavities with 
solid quartz. To this soldering effect, largely produced by the 
quartz, the present compactness, solidity, and remarkable 
strength of the dolomyte are due. 
