FAULTS AND SHEAR ZONES. 29 
resulting in slip cleavage; (3) the elongation of the mass along the strike producing 
open joints; (4) compression, both in the direction of the dip of the slaty cleavage 
and in a direction at right angles to that, thus plicating the joints in a twofold wa\ ; 
(5) the infiltration of Si0 2 and CaC0 3 in alternation into the open joints, resulting 
in banded veins; (6) the solution of the CaC0 3 by carbonic and organic acids and 
the crumbling away of the Si0 2 in consequence of exposure by erosion. Such a 
slate shows what complex structures successive crustal movements may produce in 
ordinary sediments, as well as how fatal such movements may be to the commercial 
value of slate. These joints were probably first formed by elongation, and thus are 
different from ordinary joints. 
FAULTS. 
Faults of no great magnitude are common in slate regions. The fault plane is 
frequently a cleavage or a joint plane. From the shear involved in slaty cleavage 
reversed faults are more frequent in slate than normal ones.« Minor faults from the 
western Vermont region are shown in PI. IV, C, D. One of these is a normal fault, 
the part overlying the fault plane having slidden down; the other is a reverse fault, 
the similar part having been forced up. A microscopic section across the reverse fault 
shows the sharp bending of the beds at the fault plane, and the deposition of a thick- 
ness of one-sixteenth of an inch of vein matter in bands along that plane. This 
matter consists of chlorite, calcite, and quartz. 
Some of the slate flagstones in the village of Granville, N. Y., the exact source of 
which could not be ascertained, are full of small faults, which come out finely in the 
rain, and show how much secondary compression there must have been at that 
locality. 
Faulting may occur within a slate mass in two directions at right angels to one 
another. & 
The dropping of a block of some superjacent valueless strata into a mass of com- 
mercial slate between two fault planes is a possibility that should not be overlooked 
in slate quarrying. 
SHEAR ZONES (HOGBACKS). 
These terms, as well as Knickungsebenen, Querknicke, zones of shearing, apply to 
one and the same feature. Rosenbusch, c Brogger,^ Reusch, e and Turner/ describe 
it from Norway, Saxony, Alsatia, and California. Its occurrence in the slate belt of 
eastern New York and western Vermont is fully described on pages 31, 44. These 
writers explain it as an angular plication, or a series of such plications, due to shear- 
ing pressure on somewhat rigid material. In places the pressure was great enough 
to produce a slight faulting on either side of the deflected portion or zone. Very 
rarely a cleavage foliation is produced within the zones. Mr. Turner's term abbre- 
viated to shear zone affords a convenient designation, with the understanding, how- 
ever, that in this sense it applies only to sedimentary rocks. 
The term " hogback" is used by coal miners to describe a sharp rise in the floor 
of a coal seam. The propriety of its application in slate quarries is not so obvious. 
It is used there to designate peculiar bends or fractures, which consist of two angular 
bends in opposite directions and near each other, traversing a mass of slate. These 
aSee Herbert, E. J., Reversed faults in bedded slates. Geol. Mag. N. S. Dec. II, vol. 4, p. 441, 1877. 
feSee a ease of this in the Hudson slates, of Rensselaer County, Thirteenth Ann. Reptr. U. S. Geol. 
Survey, 1894, PI. CI., p. 320, fig. 26. Also, in this connection, A faulted slate slab, by J. J. H. Teall: 
Geol. Mag., Dec. HI, vol. 1, PI. I, London, 1884. 
cDie Steiger Schiefer, 1877, p. 95. 
dBrogger, Die silurischen Etagen 2 u. 3 im Kristiania Gebiet auf Eker, p. 216, fig. 31, Kristiania, 1882. 
eReusch, H., Die fossilienfuhrenden krvstallinisehen Schiefer von Bergen in Norwegen; German 
translation bv R. Baldauf, pp. 52, 53, fig. 35; and p. 38, fig. 23, Leipzig, 1883. 
/Turner, H. W., Further contributions to the geology of the Sierra Nevada: Seventeenth Ann. Rept. 
U. S. Geol. Survey, p. 662, fig. 22, 1896. 
Bull. 275—06 3 
