"Slate Binders'" of "Pittsburg" Coal Bed. — Gresley. 359 
As to the mottling. I can best describe this by saying the 
rock is rudely but obscurely brecciated or f ragmental-looking, 
often grading into a marbly or streaky vein-like, vitreous as- 
pect, as though a variety of fragments of shale, etc., of different 
tints had been in a semi-liquid or run-together state, producing 
a slaggy or drawn-together, kneaded aspect. Some parts of 
the rock are much darker than others. There seems to be 
considerable differentiation in coloring and in shapes and 
sizes of the blotches; the individual blotches are even varie- 
gated, spotted, streaked, etc. In general, the appearance, as 
revealed by a pocket lens, when the rock is wet, and in a 
strong light, reminds one somewhat of the structure of ordi- 
nary serpentine. Pyrites occur in tiny nests or aggregates of 
crystals scattered through the groundmass. 
A polished surface of a fragment viewed at right angles to 
the bedding planes shows the character of the mottling or 
marbling shown in fig. 1. 
Pig. 1. 
Fig. 2. 
Viewed in the direction of the stratum, i. e. a transverse 
section appears as in fig. 2. Thus it will be seen that the 
blotches are several times longer in the horizontal direction 
than they are vertically. 
The fossils do not seem to pay any attention to the varie- 
gation, nor does the texture of the rock vary with the blotch- 
ings. It would appear therefore that the mottling is the re- 
sult of chemical action among the particles produced since or 
during consolidation of the deposit; though one occasionally 
finds a specimen imitating variegated breccia t<> a degree 
which even suggests uncertainty about the mottling after all. 
That the particles composing the shale were largely derived 
from feldspars is tolerably certain; and the mottling process 
may even be still going on. Of course the layers of shale 
originally possessed horizontal limits, and. if they be regarded as 
