120 
GEOLOGY OF THE THIRD DISTRICT. 
15. CATSKILL SHALY LIMESTONE. 
Delthyris Shaly Limestone, and Scutella Limestone of the Reports . 
(Part of No. 6. Pennsylvania Survey.) 
26 
No. 1. Big-ribbed delthyris (D. macropleura). No. 3. Singular atiypa (A. singularis). 
2. Smooth atrypa (A. laevis). 4. Medial atrypa (A. medialis). 
The present name of this rock is taken from Catskill creek, in the first district, near the 
town of Madison, Greene county, by the side of the railroad, where for a long distance it is 
exposed to great advantage for examination. The name is objectionable, but it is no easy 
matter to find one in the State which will be less so. 
In the first district this rock is a thick mass. The lower part is usually a mixture of dark 
bluish-grey shale, which generally changes to a drab color, and of a fine grained blue lime¬ 
stone. The upper part, at Catskill creek and the Helderberg, is a very light grey coarse 
crystalline limestone, which abounds in a discoidal shaped fossil, about two inches in diameter, 
resembling a scutella, but which appears to be the pelvis of an cncrinite. The difference 
between the upper and the lower part of the mass in the first district, gave rise to the two 
divisions into which it was divided in the annual reports ; the encrinal pelvis existing in the 
upper part of the mass, and in the first district, in great abundance. It is remarkably prolific 
in fossils not only as to individuals, but of different kinds as to species, genera, etc. ; the 
greater number of which are characteristic of it. 
In the third district the rock is not so thick, the shale diminishes in amount, and disappears 
in Herkimer county; but the limestone portion continues to the east end of Madison county, 
beyond which it has no existence. Much ihe best development of this rock exists in the first 
