VEINLETS IN THE SILURIAN AND DEVONIAN 59 
ing increase in silica, but all of the chert examined contains numer- 
ous inclusions of calcite in the form of rhombohedral crystals 
(0.05 mm. and less in diameter), somewhat larger than the similar 
rhombs in the limestone. 
STRUCTURAL FEATURES 
The rock strata have been disturbed only slightly since their 
emergence from the sea. In general, the dip is toward the south 
at an average inclination of 7 to ro m. per kilometer, but in a few 
places, because of gentle folding, there is locally considerable 
variation from this average. 
Jointing is well developed throughout the area, and is probably 
due chiefly to the adjustment of strains resulting from folding and 
tilting. Appreciable openings are not found along these joints 
except near the surface, where, under favorable circumstances, they 
have been widened by the solvent action of descending surface 
water, and in such instances little or no deposition is to be observed 
on their walls. The fracturing of the rock strata seems to have 
resulted from compressive forces which would tend to prevent the 
formation of open fissures. ‘The joints cut the veins of the region, 
and are therefore, in part at least, of later origin. 
A thrust fault with displacement of a few centimeters is exposed 
in the Backus quarry, two miles north of Union Springs, and here 
the drag of the rock strata on both sides of the fault plane indicates 
that the displacement was accompanied by sufficient pressure to 
keep the fracture closed. A narrow vein of selenite follows this 
fault. Hopkins has described several thrust faults in the vicinity 
of Syracuse, the displacements ranging from a few centimeters to a 
little over a meter." 
Certain local disturbances of the rock strata, not noticeable in 
the overlying formations, may be observed in the Cobleskill lime- 
stone and the upper beds of the Salina. In places these strata have 
been pushed upward in such a way as to form low domelike eleva- 
tions on which the joints sometimes have a radial arrangement. 
A group of six or more domes may be found a kilometer southeast 
tT. C. Hopkins, ‘‘The Geology of the Syracuse Quadrangle,” N.Y. State Museum 
Bull. 177, 1914, p. 20. 
