32 SLATE DEPOSITS AND INDUSTRY OF UNITED STATES. 
, Doctor Becker" explains this structure by the alternate interference and coinci- 
dence of waves of vibration produced by shock. When the waves of vibration com- 
ing from opposite directions coincide, cleavage fractures — i. e., planes of slip cleav- 
age — will be numerous. 
Professor Van Hise regards such structures simply as the result of the concentra- 
tion or sparseness of slip cleavage. b 
In the ledge figured in PL XI, B, there are between 360 and 530 such planes to 
the inch in the cleft bands. The hard bands in the same ledge show as many frac- 
tures, but they are discontinuous or merely incipient. Both hard and soft bands 
(•(insist of stratified shale made up of quartz fragments, muscovite and chlorite 
scales, etc, but in the soft ones the pressure, has in places brought about sufficient 
alignment of the muscovite to produce aggregate polarization, not in the direction of 
the slip cleavage but in that of the bedding. There has also been a formation of 
sericite along the slip-cleavage planes, and thus the rock is becoming a schist. 
In a piece of sericite schist from the mass east of Rupert the rock consists of alter- 
nating strips with and without slip cleavage, those without being wider. The slip- 
cleavage planes meander about and run into each other, though having a general 
parallelism. The plication is much more intense and irregular in the cleft strips 
and also appears to be more sericitic there. 
Fig. 2. — Cleavage banding showing slippage of bands; brook iuGorham, Poultney, Vt. 
In all these cases the fractures are the result of the plications. 
It seems probable that the shear /ones represent but a variation of the process 
involved in the cleavage bands. The force bent and crushed the slate in the bands 
instead of producing very numerous planes of slip cleavage within them. 
VEINS. 
Quartz veins or " flints," as the quarry men call them, are a striking feature of 
slate as well as a cause of perplexity in quarrying. They appear at the most unex- 
pected points, frequently ramify very irregularly, and disappear as suddenly. PI. V, 
B, from the old quarries at James ville, N. Y., shows a good type of such veins of 
a Finite homogeneous strain, flow, and rupture of rocks: Bull. Geol. Soe. Am., Vol. IV, Jan., 1895. 
On page 16 he says: " Thus there seems sufficient reason to believe that a pressure very rapidly 
applied, producing primary ruptures attended by shock, will be immediately followed by secondary 
ruptures in the same direction as the others at intervals dependent upon the wave length of the 
impulse. In much the same way a high explosive shatters a rock far more than black powder. A 
phenomenon of which no explanation has been offered in this paper is that of thick slates and of 
those flags which are to be considered as very thick slates. These, though cleavable in a certain 
thinness, are not capable of further splitting. Such rocks indicate a flow which is not uniformly dis- 
tributed through the mass, but on the contrary passes through maxima at intervals corresponding to 
the thickness of a slate or flag. It is possible that at the inception of strain such masses were in a 
state of tremor so intense that the interference of waves determined surfaces along which flow began. 
These surfaces would be weakened by the flow, and further strain would he distributed among them 
rather than over the intervening solid sheets. Effects of a similar kind are produced on a pile of 
sheets of paper, such as ' library slips,' resting on an inclined cloth-covered table which is jarred by 
rapid blows." 
^Sixteenth Ann. Kept, r. s. Geol. Survey.pt. 1, Principles of North Am. pre-Cambrian Geology, 
1896, pp. 662-664. 
