214 
GEOLOGY OF THE FOURTH DISTRICT. 
separates the Moscow shale from that below it. Several of the fossils of this rock are pecu¬ 
liar to it, being unknown elsewhere. The same is true also of the fossils of the Encrinal 
limestone, before spoken of in the Hamilton group. 
Concretionary Structure of the Tully Limestone. 
Near Big-stream point, on Seneca lake, where this limestone rises above the level of the 
water, the lower strata exhibit a concretionary structure, and some portions are actually sepa¬ 
rated into regular spherical concretions. The sketch below is a section at this place. 
91 . 
This structure seems to be owing to a meagre supply of the material, which when spread 
over the bottom was insufficient to form a continuous stratum, and collected into these spheri¬ 
cal and concretionary masses. Similar effects seem to have occurred in other cases, parti¬ 
cularly in some of the shales where a small portion of calcareous matter has aggregated itself 
into spherical forms, presenting a continous course of these, which hold the same position in 
the strata as if a thin layer, once continuous, had been rolled up into spheroids. 
Localities. —A few localities will suffice for acquiring a knowledge of this rock in the Fourth 
District. The western shore of Cayuga lake south of Ovid, and the eastern shore of Seneca 
lake, from Hathaway’s landing southwards, will give good opportunities of examination. The 
western shore of Seneca lake and the outlet of Crooked lake are also equally good localities. 
At the village of Bellona, in Ontario county, it is well exposed in the bed of a small stream, 
and contains more of its fossils than at any other locality. There are some other points of 
minor interest, but they afford the observer nothing new, or of different interest from those 
enumerated. 
Thickness. — The greatest thickness of this rock within the district is but sixteen feet, and 
in most of the localities it is less than ten feet thick. It gradually diminishes westward, and 
from Canandaigua lake westward its place is occupied by a calcareous band of three or four 
inches thick. 
Mineral contents. — The rock contains no mineral of interest. The cavities of fossils are 
often lined with calcareous spar, and in a few cases some small crystals of sulphate of baryta 
have been obtained in similar situations. 
