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[ Assembly 
ed under the head of Tompkins county. Near the village of Penn- 
Yan the same group is perhaps better displayed than in any other place 
examined. Its fossils being so distinct and peculiar place this in an in- 
dependent group, or it may be joined with the black shale below, and 
constitute a characteristic group composed of the two members. 
The rocks of this group are the highest in Seneca county, extending 
southward into Tompkins, where they are succeeded by another group, 
which will be described under the head of that county. 
ONTARIO COUNTY. 
In this county we find the same rocks, and in the same general order, 
as in Seneca. The " Saliferous Group of Onondaga^'^ occupies that 
part of the county which lies north of a line drawn from a point one 
mile north of Oak's corners, in the town of Phelps, along the course of 
the Canandaigua outlet to Manchester, and thence west to Victor. 
The vast accumulation of alluvial, however, leaves the rocks exposed 
but at few points, which are mostly along streams. In a few cases, 
what externally appear to be alluvial hills are isolated masses of gypse- 
ous marl, &c. deeply covered with alluvium; and in some instances 
small masses of gypsum have been found in these, much above the or- 
dinary level of the valleys and surrounding country. Admitting that the 
gypseous mass originally held a higher elevation, and equal to that of 
the alluvial hills, before the intermediate portions were transported 
southward, the fact proves the vast quantity of matter removed by allu- 
vial agency. 
This group of rocks is much better exhibited in Ontario than in Se- 
neca county; having a greater extent, and in one or two places develop- 
ing a different character. The gypsum here occupies three distinct 
ranges; the northern or lowest appearing upon or just north of the 
county line, and the other two within the county. In the first the gyp- 
sum is associated with a gray marl, which reposes on a green marl con- 
taining no gypsum. The gray marl exhibits iron stains and decompos- 
ing pyrites; and pyrites is also found with the plaster which occurs in 
small, irregular masses, having a granular or crystalline texture, and 
frequently accompanied by selenite. Masses of selenite are very abun- 
dant in the marl, varying from the size of a walnut to several pounds 
weight, isolated as regards themselves, and having no connection what- 
ever with the larger masses of gypsum. In appearance and mode of 
formation they are very analagous to those found in the marly clays of 
the Tertiary formation. The limpid selenite often embraces a small 
