CLEAVAGE BANDS. 31 
flexures may and often do merge into fractures, and the slate between the two planes 
of fracture is broken into small fragments. The two bends or fractures may be 
anywhere from one-sixteenth of an inch to 4 feet apart. At one of the old Middle 
Granville, N. Y., quarries a slate surface shows within a space of 4 inches four " hog- 
backs" varying from one-sixteenth to one-fourth inch in width. Another has six in 
a space of 6 inches. At one of the Arvonia, Va., quarries a mass 5 feet thick is trav- 
ersed by numerous shear zones. As they generally traverse the cleavage diagonally, 
the blocks of slate adjacent to them come out in triangular form, and thus occasion 
much waste. The strike of some of the shear zones in western Vermont is parallel 
to that of several of the dikes, and both may have been formed under the same si ress. 
Figs. E, G, I, on PL XXIII, and figs. L, U, and V, on PL XXIV, show the relations of 
"hogbacks" at several quarries. Fig. A, on PL VII, shows the microscopic structure 
of the "hogback" of Fig.V, PL XXIV. The section was made where the bends had 
not as yet developed into complete fractures. The entire width of the "hogback " is 
nine-tenths of an inch. Between the two sides is a system of cracks crossing the 
cleavage at an angle of 15°; another system crosses the cleavage at 25° and extends 
beyond the side. This may be the original bedding. Both of these systems of cracks 
are rilled with secondary calcite. Another set, likewise filled with it, crosses the 
cleavage here and there about at right angles, but in zigzag (see figs. B, C, D, PL 
VII). These, farther on in the quarry, become continuous fractures. The second- 
ary fractures within the zone probably result eventually in breaking up the slate, as 
it occurs usually in fragments within a fully developed shear zone. The observations 
at this quarry were: Strike of bed N.=b, dip 30° E.; strike of cleavage N.=b, dip 42° 
to 45° E. ; strike of shear zone N. 35° to 40° E., dip 65° NW. In this case the strike 
of the shear zone corresponds to that of the diagonal joints of the region and of 
a number of the dikes. Figs. E to L, on PL VII, illustrate the development < >f a 
shear zone still further. In fig. L the diagonal fractures are filled with quartz, and 
a slip cleavage (false cleavage) also occurs. Other observations of "hogbacks" are 
given on the quarry tables, pp. 102, 103. 
CLEAVAGE BANDS. 
While characteristic of slate regions, cleavage bands do not occur or have not yet 
been reported in commercial slate. They would be quite as objectionable as are 
shear zones. 
PL XI, i?, is from a photograph taken at a locality in Rupert, Vt., found by Mr. 
L. M. Prindle. Cleavage banding is of frequent occurence in the Vermont and New 
York slate belt and in the schist mass east of it, although not always as well shown 
as at this point. It resembles the shear-zone structure just described, but presents 
further stages. In PL XI, B, the rock is divided into alternate bands of hard, uncleft 
quartzose shale and of bands of very finely cleft shale. The bedding zigzags across 
both bands. The material of both bands was originally identical. The present dif- 
ferences are the result of a difference in the amount of motion — i. e., of slip cleavage— 
along alternating strips of rock and of the consequent difference in resistance to ero- 
sion. There is also a difference in color, some infiltration of limonite having taken 
place along the more highly cleft bands. 
A similar structure in purple slate is shown in fig. 2, but here the bands them- 
selves have slipped. « Small beds of green slate indicate the course of the stratifica- 
tion and show the amount of slippage suffered by the bands. 
In the Poultney* River, about a mile east of East Poultney, certain hard, bright 
green and purple slates near the schist mass show the cleavage banding well. In 
breaking up such a rock the denser uncleft parts come out in slab-like blocks, the 
larger surfaces of which lie transverse to the bedding. 
a See also Sixteenth Ann. Rept. U. S. Geol. Survey, pt. 1, 1896, pp. 561-564, figs. 88-91. 
