22 SLATE DEPOSITS AND INDUSTRY OF UNITED STATES. 
sisting largely of calcite. This explains why the beds are so clearly and yet so deli- 
cately brought out on the joint face. The original sediments had varying amounts of 
calcareous material in them. The carbon dioxide brought down from the atmosphere 
by the rain has, as it were, carried away the more calcareous parts, leaving the less 
calcareous ones in relief. 
In many of the eastern New York and western Vermont quarries change of color 
alone is an indication of the passage from one bed to another. This change may be 
gradual or abrupt. But color is not an infallible guide, as the red slate sometimes 
passes into the green along the same bed, and there is no reason why the Cambrian 
purple should not likewise pa.^s into the green of the same formation. 
In cases where there are no fossil impressions or intervening beds of very different 
material or partings or slight changes in the composition of the slate itself, produc- 
ing changes of color or different degrees of erodibility, the course of the bedding can 
sometimes he made out in a thin section cut transverse to the cleavage when exam- 
ined under the microscope. There may be an occasional arrangement of the par- 
ticles parallel to the original bedding or an extremely minute bed of other material, 
or lines of different particles may cross the cleavage foliation. Fig. C on PL VI 
illustrates cases of this kind. 
Gosselet" gives some remarkable instances of intense and complex folding of beds 
of slate on a large scale. These great folds are very acute and overturned. In some 
places shafts have been dug through other rocks in order to reach underground por- 
tions of synclines and anticlines and quarry the slate. Slate quarrying in the Arden- 
nes thus resembles coal mining in a region of intense folding. 
SLATY CLEAVAGE. 
The causes and the structure of slaty cleavage have already been discussed under 
the headings "Origin of slate" and " Petrographie characters." 
In most slate regions cleavage is not coincident with bedding. Its relations to' 
bedding are illustrated in figs. 6-9 and Pis. Ill, V, A; XII-XIV, XVII-XIX, 
XXIII-XXV. The lowest inclination of slaty cleavage in slate districts visited by 
the writer is 5°-10° and occurs in Pennsylvania (see Pis. XIII, XIV).'' The lowest 
in the eastern New York and western Vermont belt is 20°. Where the cleavage 
so nearly approaches horizontality as it does in the first instance its position is prob- 
ably due in part to a secondary crustal movement and the occasional curvature of 
both joints and cleavage there also point to such a movement. 
Amount of compression in the formation of slaty cleavage. — Sorby calculates on a small 
bed of intensely plicated sandy slate, inclosed in ordinary slate, that the amount of 
shortening by plication was about 75 per cent, and reasons that the clayey material 
of the slate itself must therefore have been compressed to the same extent. This is 
the only way in which the amount of compression actually suffered by a mass of slate 
could be computed. But this calculation does not take into consideration the elonga- 
tion of the slate in the shearing. 
Relation of cleavage dip to dip of inclosing hard beds. — Gosselet brings out the fact 
that where a bed of slate lies between beds of a hard rock like quartzite there is a 
constant geometrical relation between the degree of the cleavage dip of the slate and 
of the dip of the beds of quartzite. The same thing is true, as he shows, in the hori- 
zontal relations between the strike of the cleavage, that of the quartzite bed, and the 
direction of movement: 
Unless the ancient shore yielded to the pressure from the south the lower beds must have had a 
tendency to rise against that obstacle, to slide as a wedge between the obstacles and the overlying 
beds. The slaty material inclosed between the two beds of quartzite is thus pushed upward in a 
direction which is the component between a vertical line (i. e., vertical to the horizon) and the 
oblique movement of the wall (i. e., along the surface of the slate bed). Cleavage will be developed 
along that component, and we actually find that it dips 40° while the bed dips 27°. 
aL'Ardenne, p. 41, fig. 7, Les rapports de Ste. Marie avec les Tr<5sfosses. Les schistes de Fumay, 
PI. Ill, fig. 1: Bond dans les sehistes de Fumay. 
&See also Thirteenth Ann. Rept. U. S. Gcol. Survey, PI. XC, showing a slate quarry near Lebanon, 
X. V., witli cleavage dipping 10°. 
