No. 50.] 
423 
material, a continuous stratum three or four feet in thickness. This 
change is seen in many cases near the thinning out of a mass ; the s}ip- 
ply of matter diminishing till it is traced only by distant nodules o^.' con- 
cretions.* 
Near Geneseo, Moscow and Mount Morris the black shale is charac- 
terized by the presence of great numbers of Posidonia, which is almost 
the only fossil it contains. Two species of Lingula, an Orbicula, and 
an Orthis are characteristic of this shale in other places. 
In several places this rock is traced for several miles from its nor- 
thern outcrop on the hills, to its final disappearance south in the bottom 
of the valleys. This is owing to the circumstance that the valleys are 
excavated in the direction of the dip of the rock. Its final disappear- 
ance is in the bed of the Genesee, two miles south of the Mount Morris 
bridge. 
Cashaqua shale. — A hundred feet of this rock is exhibited at the 
gorge at Mt. Morris, limited below by the black shale just described, and 
above by the Gardeau group. It also appears in many ravines in the 
south and southwest part of Leicester ; in the vicinity of Mount Mor- 
ris, in the Cashaqua creek, whence it takes its name ; in the ravines on 
the east of the valley, and at a higher elevation southeast from Geneseo 
and approaching nearly to the village. 
The Gardeau and Portage groups already described are the southern 
rocks of the county. These are seen in the deep gorge of the Ge- 
nesee, and in almost all the ravines and water courses of the southern 
towns. Among numerous localities as we approach Dansville may be 
mentioned. Stony brook in Sparta where several hundred feet of these 
rocks are exposed. The shale in the upper part of this ravine has been 
ground and used as plaster. Several years since, this gorge was swept 
by a freshet which covered many acres of land in the valley, and changed 
the course of the stream. 
* Those masses which, are properly termed septaria, consist chiefly of compact car- 
bonate of lime, aggregated in a spherical form, and divided in various directions by 
seams of crystalline matter, carbonate of lime, sulphate of baryta, &c. The masses 
often inclose cavities containing other crystalline substances, and also petroleum. 
Again other masses and often part of the same course are not crossed by seams ; but 
are nevertheless the result of the same laws, differing only in this, that after aggrega- 
tion, no part was separated by crystallization. 
