10 SLATE DEPOSITS AND INDUSTRY OF UNITED STATES. 
Becker's experimental studies were directed to determining the mathematical 
character of the deformation undergone by a plastic but crystalline mass under pres- 
sure. For this purpose he pierced -a block of ceresin with needle holes at regular 
intervals in both a vertical and a horizontal direction, drawing a thread smeared with 
coloring matter after the needle, and subjected the block to powerful vertical pres- 
sure. The curves then assumed by the colored lines enabled him to determine the 
position of the axes of the strain ellipsoid with relation to the direction of pressure. 
The result of this and of other experiments described in the same paper is that — 
cleavage does not coincide even approximately with the direction of the major axes of the strain 
ellipsoid. Neither does the cleavage correspond to the position of the major axes of the strain ellip- 
soids at any previous stage of the strain." 
In other words, slaty cleavage is not perpendicular to the smallest axis of the fetrain 
ellipsoid, but makes with "that axis an acute angle equal or greater than 45° and 
increasing as the strain grows greater." Both the experiments and the reasoning 
seem simple and clear. 
Leith, following Van Hise, states that— 
Cleavage is always tending to develop normal to the greatest principal stress, but its final position 
may or may not he inclined to the greater stress, depending upon the nature of the strain./' 
This question of the relation of the dip of slaty cleavage to the direction of pres- 
sure is not a purely speculative one, but bears directly upon problems in field geol- 
ogy. Thus where cleavage is nearly horizontal it is probable that a secondary crustal 
movement must have diminished the original inclination of the cleavage, and con- 
sequently the whole attitude of the folds. Were the limiting angle at which slaty 
cleavage forms established, then, in the case stated, a maximum figure for the angu- 
lar displacement of the cleavage could be obtained. 
There is some vagueness in S( i» -u t i 1 i< • literature, as well as in popular conception, as 
to what constitutes a slate. Sorby drew this distinction: 
When a section [of a fine-grained slate] cut at right angles to the cleavage is rotated in polarized 
light it becomes, over nearly the whole surface, very bright, and much darker at different azimuths, 
like a doubly refracting crystal, whereas there is little or no such change in the case of true clay 
slates of the normal granular type containing much kaolin and very little mica.'' 
Micaceous luster is not a satisfactory criterion, for some slates, like those from 
Monson, in Maine, are almost lusterless and yet possess a brilliant aggregate polariza- 
tion and are very sonorous. They are mica-slates (phyllite-slates), as are the Brown- 
ville slates, from the same State, which have a micaceous luster. 
An interesting transition from a shale to a mica-slate, described on page 81, occurs 
at Lehigh Gap, Pennsylvania. The new muscovite (sericite) has begun to form across 
the bedding of the shale without as yet any change of orientation in the clastic, par- 
ticles. In a specimen of shale from Rensselaer County, N. Y., d the incipient align- 
ment of clastic muscovite across the bedding is well shown in thin section, and slip 
cleavage seems about to be set up. Some specimens of clay-slate from Martinsburg, 
W. Va., show a slaty cleavage without any matrix of muscovite whatever, but others 
show the beginning of one by a faint aggregate polarization. These cases suffice to 
show that all transitions from a shale to a clay-slate and to a mica-slate occur. 
While it is supposed that mica-slate is the product of a less intense metamorphism 
than schist e — and it is also assumed that the process by which slaty cleavage was 
produced was marked by great uniformity, and also took place slowly — there are as 
yet neither observations nor experiments for a perfectly satisfactory reply to the fol- 
a Experiments on schistosity and slaty cleavage: Bull. U. S. Geol. Survey No. 241, 1904, p. 21. 
6 Rock cleavage: Bull. U. S. Geol. Survey No. 239, 1905, p. 138. 
"On the structure and origin of nonealcareous stratified rocks: Quart Jour. Geol. Soc. London, vol. 
36, p. 76. 
dSee Bull. U. S. Geol. Survey No. 242, PI. II, B. 
e Van Hise, op. cit., p. 894: "When the depth is not great and the mass-mechanical action is not 
very severe, slates are likely to form. When the depth is greater and the mass-mechanical action is 
severe, schists or gneisses are likely to develop." 
