CHEMUNG GROUP. 
257 
the strata present lines of diagonal deposition. This structure has been before illustrated, 
under Medina sandstone and Portage group. 
Concretions , and concretionary structure of strata. — In the shales of the lower part of the 
group, well defined concretions frequently occur in great numbers. These are highly cal¬ 
careous, and present the common appearance of those in the groups below, being divided by 
seams of crystalline calcareous matter, and contain cavities often partially filled with bitumen. 
Towards the higher strata of the group, the shaly sandstone often becomes concretionary, 
the laminae folding entirely or partially around a nucleus. These present various degrees, 
being often nearly spherical, and becoming less and less perfect, till they present only strong 
and abrupt undulations in the laminae of the stratum. Such structure usually occurs in the 
micaceous shaly sandstone, and more strongly marked when such a stratum passes into shale. 
The following section, near Painted-post, illustrates this structure : 
113. 
A similar structure is observable in the exposed cliff at Chemung narrows, and near El¬ 
mira. The laminae of these concretionary masses preserve the same lines of division as where 
the strata are plain, and they separate readily* 
Spheroidal desquamation. — Fragments of the sandstone, of some parts of this group, after 
weathering upon the surface, desquamate in concentric laminae to the depth of the weathering. 
This seems due to partial decomposition, which causes the separation, and obliterates the 
lines of deposition. The blow of a hammer will often separate these concentric layers to the 
thickness of an inch, while the apparent nucleus presents a different color, and the parallel 
lamination of the mass is often visible. The influence of the weathering is very perceptible, 
and is evidently the cause of the separation. 
Ripple-marks. — The thin-bedded sandstones and sandy shales of this group are often 
ripplemarked ; but from the frangible nature of the strata, it is not often easy to preserve them. 
The following woodcut is from a beautiful specimen, from Chautauque county, where the 
undulations of the ripple are perfectly preserved : 
* These separated laminae form large troughs, bowls, etc., which are frequently used in this part of the country as substi¬ 
tutes for more perishable articles of the kind. At a farm-house, I have seen them used for wash-hand basins ; and in other 
places, I have seen the swine and barn-yard fowls feeding from similar forms. 
[Geol. 4th Dist.] 33 
