168 
GEOLOGY OF THE FOURTH DISTRICT. 
exhibitions of the strata than others. The numerous quarries mentioned in Seneca county, 
are all interesting localities. The bed of Flint creek at Vienna, the outlet of Canandaigua 
lake, and the bed of Mud creek, all offer interesting points for examination. At the village 
of West-Mendon, the strata are well exhibited, though few fossils occur. To the south and 
west of the village of Caledonia, and the whole distance between this point and Le Roy, there 
is a fine exposure of the rock. The bed of Allen’s creek at Le Roy, as far as the falls, affords 
perhaps the best exhibition of the strata in the district.. At Clarence Hollow, this rock con¬ 
tains a few fossils not elsewhere seen. In the vicinity of Williamsville, and at several points 
between this place and Buffalo, to the south of the turnpike, the upper part of this rock is well 
developed. It is peculiarly interesting from containing numerous fossils of the genera Stro- 
phomena, Delthyris, and Pleurorhyncus. At Black Rock, the lower part of the mass is very 
fully developed, and presents many interesting fossils. 
The places enumerated will give a very perfect idea of the character of this rock; indeed, 
two or three of these localities are sufficient to present its most characteristic features. 
Thickness. — The point of greatest thickness actually measured is on Allen’s creek, where 
it is seventy-one and a half feet. At the eastern extremity of the district, the thickness can¬ 
not be more than half this, and at some points I have estimated it at thirty feet. At Black 
Rock there is about twenty-five feet laid open to view; but the higher part of the rock is not. 
visible, and from the deep alluvion covering it farther east, it cannot be correctly estimated. 
The increased thickness in many instances seems due to the augmentation of the hornstone, 
which is exceedingly variable, often being collectively less than six feet, and again amounting 
to four or five times as much. 
Mineral contents of the Corniferous limestone. — The mass presents few simple minerals, 
besides those which form an essential part of its composition. The hornstone sometimes 
passes into chalcedony, and crystallized quartz is rarely found. In the cavities in this rock 
at Black Rock, dogtooth spar sometimes occurs. Fluid bitumen, or petroleum, occurs in the 
same situation, usually filling the cells of the Favosites, from which, when broken, it often 
exudes in considerable quantity. 
Springs. 
This is one of the most interesting rocks in regard to the production of springs. As before 
described, the rock is crossed by vertical joints in two directions. The planes of these joints 
are usually separated several inches, and often several feet toward the northern margin of the 
rock ; and by enlargement within, they sometimes form the entrance to a cavernous expansion. 
The soil above being usually thin, the water readily finds its way into these fissures, and passes 
off beneath. From this circumstance, it is often difficult, or nearly impossible, to obtain water 
by digging wells, unless the rock is blasted to great depth. 
The water passes downward till it comes to an impervious stratum, where it is held up, and 
rising in the fissures till it can find some outlet, gushes out at the northern edge of the escarp- 
