392 
[Assembly 
The two groups just described, occupy a thickness of more than 
1,000 feet, and are interposed between the Cashaqua shale and the 
Chemung group. Indeed, if we consider the Chemung group as com- 
mencing with the occurrence of its characteristic marine fossils, then 
several hundred feet more of rocks may be noticed as intervening be- 
tween the upper Portage rock and that group. 
The rock succeeding the upper Portage rock, consists of greenish 
olive sandy shale, or very shaly sandstone, the whole mass of a homo- 
geneous appearance, and never slaty. The only fossil seen in this 
rock is a species of fucoides with a striated surface, and these are by 
no means numerous. This is succeeded by a dark, nearly black, 
sandy, highly micaceous shale, with septaria. It contains iron py- 
rites, and where exposed, is of an iron rust colour, externally. Some 
thin masses of gray sandstone are interstratified, which contain fossils 
referable to the Chemung group. The Chemung group occupies a 
large area in the southern counties, and with the exception of a small 
thickness above, forms the highest mass in these counties. 
The Gardeau and Portage rocks are well entitled to the place of dis- 
tinct groups, both from their general mineralogical character as well as 
from their fossils, both differing from rocks above and below. In the 
Portage group, a single specimen of Cyrtoceras has been found, and in 
the upper part of the Gardeau rock, a few of the fossils of the Casha- 
qua shale were found ; but this must be considered accidental, and 
forms no character by which the rocks may be identified. 
In some of the upper sandstone strata of the Gardeau group, there is 
an apparent tendency to concretion, though the masses are distinctly 
laminated. The upper surface of the stratum appears as if it had been 
compressed while soft, and exhibits numerous irregular concave depres- 
sions, the stratum thinner at this point, and the laminae apparently 
more closely compressed. There occur in the shale of this part of the 
group, nodular or concretionary spherical masses, of from six to twelve 
inches in diameter ; these masses usually consist of a solid nucleus, 
and at the outside to the depth of two or three inches, radiating out- 
wards ; when broken, these radiating portions present somewhat the 
appearance of the lignilites or magnesian striations in the hydraulic 
limestone. Some similar cause may have produced these, but it can- 
not be entirely the same. The same substance sometimes takes the 
form of thin interrupted courses in the shale, and the concretions are 
generally much flattened. 
