24 SLATE DEPOSITS AND INDUSTRY OF UNITED STATES. 
Effect of frost on, cleavage. — As all slate quarrymen know, repeated freezing and 
thawing is disastrous to the cleavability of roofing slates. The material must be 
split fresh from the quarry. In order to ascertain, if possible, what difference in 
microscopic structure freezing and thawing produced this experiment was tried: A 
specimen was obtained from the quarry early in the winter fresh and unfrozen, and 
was kept moist in a moderate temperature until severe weather set in. It was then 
broken into two equal parts, one of which was kept moist indoors, the other exposed 
on the sill of a north window for a week, during which the temperature went down 
to 10° below zero F. This part was then thawed out over a furnace register. Both 
frozen and unfrozen pieces were after some accidental delay sliced and examined 
microscopically. The whole texture of the frozen slate was found to be perceptibly 
closer than that of the unfrozen. The test would have been more satisfactory had 
the thin sections been made at once. The loss from freezing and thawing is so con- 
siderable that means to recover it have been sought, and a process has recently been 
patented for restoring the fissility of slate by the use of liquid air." 
Curvature of the cleavaqe. — As far back as 1839 De la Beche called attention to the 
curvature of cleavage planes when approaching a bedding plane. & Baur in 1846 
observed S-like cleavage foliation in Germany, and describes certain slates which 
were so much curved as to be fit for use only on the roofs of towers, but he does not 
explain whether this curvature was parallel or transverse to the bedding. c John 
Phillips, in his British Association Report, ascribed these curvatures to the differing 
density of the beds. rf Harker in some cases attributes it to a gradual change in the 
texture of the beds. e In other cases a secondary motion is called in to explain it./ 
Curvature of the cleavage is not uncommon in the Lehigh and Northampton County 
quarries in Pennsylvania, where it is plainly not dependent upon change of texture 
(see pp. 76, 83). Slates from the beds so affected are also used for roofing towers. 
Phillips 9 gives a figure, the original authorship of which is not mentioned, repre- 
senting the cleavage surface of a piece of slate in which gently plicated ribbons are 
shown. A normal fault crosses the piece diagonally, displacing the beds. The 
cleavage surface also shows the "flexuous" lines of a third foliation oblique to the 
cleavage. Finally two small calcite veins cross the primary cleavage, the plicated 
bedding, the plicated secondary cleavage, and also the fault plane. The specimen 
thus bears traces of at least five, if not six, motions. 
SLIP CLEAVAGE ("FALSE CLEAVAGE.' 1 ) 
Several writers — Sedgwick, ^ Phillips, * De la Beche, J Zirkel,^ Loretz^ — describe 
a striation or an extremely fine plication that appears on the cleavage surfaces 
of some slates. This is the "bate" or "false cleavage" of quarrymen. Two sys- 
aBy W. A. McLaughlin, Delta, Pa. 
b Report on the Geology of Cornwall, Devon, and West Somerset, p. 620, fig. 31, London, 1839. 
cUeber die Lagerung der Dachschiefer, etc., pp. 392, 393, fig. 9. (For full titles of works cited see 
Bibliography, pp. 138-145.) 
dOp. cit., p. 384, fig. 23. See also Jukes: Quart. Jour. Geol. Soc, vol. 22, p. 359, 1866. That the angle 
and amount of cleavage change with the density of the roek whs shown by Phillips in 1828. See alsc 
Harkness, op. cit., 1855. 
«On slaty cleavage, etc. 
/Hughes, T. M., quoted in Lyell's Students' Elements, 7th ed., pp. 53, 573, fig. 625, 1871. 
Q Report on the Geology of Cornwall, etc., p. 372, rig. 2. 
h "While my first observations on cleavage planes were made during long bygone years in Cumber 
land, I had hardly noticed the phenomenon of a second cleavage plane; but on many occasions 
have subsequently collected, from various parts of England, a considerable and unpublished mass o 
materials in illustration of this second plane. The second cleavage plane is generally inclined at i 
great angle to the first plane. Most beautiful examples of this double structure were seen in 1839 b] 
Sir R. Murchison and myself in the quarries of the Ardennes, where the fine, glossy surfaces of thi 
slates are frequently marked by the parallel striae of second cleavage, and the economical value o 
the slates is sometimes much deteriorated by the second plane. By a powerful reflected sunlight 
have frequently been able to trace these striae of a second cleavage on the surface of the Bangor slate 
which have been brought to Cambridge." Synopsis Classif. British Paleozoic Rocks, p. xxxv; Lor 
don 1855. Also, by same author, Trans. Geol. Soc. 1840, n. 655. 
* On a group of slate rocks, 1829, p. 1. 
iGeol. Observer, second ed., 1853, p. 588, fig. 239. 
fcLehrbuch, 1894, pp. 307, 308. 
ZTJeber Transversalschieferung. pp. 263, 264. Spurr, J. E., describes slates in Minnesota with t\v 
and three cleavages and bedding: Am. Jour. Sci., 3d ser., vol. 48, p. 159, 1894. 
