274 The American Geologist. ^''^' ^^^^^^ 
general region), nor any "river valley wash," nor any "het- 
erogenous mixture." It is a homogeneous, uniform, exceed- 
ingly fine,, non-effervescing clay, containing only debris from 
the adjacent Carboniferous strata, its nature being more that 
of a residuary clay than anything else that the writer ever 
saw. In its lawer portion, and at higher levels where the skel- 
eton was found, are masses of Carboniferous limestone, but 
the leaching and rotting have been so thorough that the cal- 
cium carbon'ate is not appreciably distributed in the matrix in 
which they lie ; but each limestone mass is surrounded with a 
coating, from half an inch to two inches thick, of its own un- 
disturbed residuum. Hence they are slippery, when wet, and 
unhandy to move or to break with a hammer. These large 
limestone masses are more and more f^xquent toward 
the face of the tunnel, being almost wanting below the silt 
layer at the entrance. The writer made a microscopical ex- 
amination of the substance below the silt layer. After repeat- 
ed decantation from a handful washed repeatedly in a jar, 
from which therefore the finest grains were cupposed to have 
been removed, he was surprised to find that the finer of the 
remaining grains and all the coarse ones, were perfectly homo- 
geneous and uniform grains of itself. These homogeneous 
grains showed, still, that they were compound, and consisted 
of aggregations of grains of an excessively fine substance. 
A number of mountings were made, in an efi'ort to ascertain 
the nature of the ultimate particles. In each instance the 
mounted grains showed still their composite structure : until, 
at last, the very dust ,that adhered to the white paper from 
which the visible grains had been shaken ofit, was put on the 
slide and covered in Canada balsam. Scattered amongst some 
still compound grains were seen, in the highest powers, the is- 
olated, ultimate grains of the substance. They appeared tO' 
be entirely of ragged fragments of kaolin, never showing 
any roundness that could be attributed to friction vmder trans- 
portation. When aggregated these fine particles interfere 
with the transmission of light, and, owing probably to the 
presence of a small amount of iron oxide, the small clusters 
appear nearly opaque except at the extreme ends of small pro- 
jections." This oxide also gives the brown stain which cliar- 
acterizes the clay in bulk. 
