SLIP CLEAVAGE. 25 
tems of such lines or plications may occur in the same slate. These are due to a 
secondary and tertiary cleavage, a slip cleavage developed upon the primary slaty 
cleavage. This consists simply of very minute plications which usually result in 
microscopic slippages or faults along which the slate easily breaks. This structure 
may show itself where slaty cleavage alone is visible or in the bedding also (see 
Pis. VI and XI, A). There is a readjustment of the slaty cleavage and the bedding 
foliation with reference to the new pressure instead of a rearrangement of all the 
particles as in slaty cleavage. The presence of "false cleavage" can be detected 
microscopically in a piece a half-inch square as certainly as from tests applied to 
pieces of commercial size. The specimen shown in PI. VI came from a quarry which 
proved a failure on account of this structure, but it was not detected until after the 
expenditure of much money. The Arizona slate, described on page 51, shows two slip 
cleavages crossing one another, as well as both bedding and slaty cleavage. 
False cleavage has received in recent years a variety of technical names: ( 'lose-joint 
cleavage, strain-slip cleavage, Ausweichungs-clivagr, fissility, fracture cleavage. Science is 
not advanced by the mere multiplication of technical terms. The term "fissility," 
being a synonym of Latin derivation for cleavability of Anglo-Saxon origin, is likely 
to be misleading. "Close-joint" and "fracture cleavage" are objectionable because 
they may be applied to jointing. The term "slip cleavage" has brevity and definite- 
ness in its favor, as well as priority over the last two. Slip cleavage is a common fea- 
ture in schist and is particularly characteristic of theTaconic region in western Mas- 
sachusetts, Vermont, and eastern New York, where it seems to be due to a continued, 
if not, in some instances at least, a secondary, crustal movement consequent upon the 
first metamorphism. Good illustrations of slip cleavage will be found in the follow- 
ing publications of the United States Geological Survey: Monograph XXIII (1894), 
figs. 44, 45, 46, 53, 56; Thirteenth Annual Report (1894), p. 319, fig. 25; Fourteenth 
Annual Report (1895), p. 537, fig. 57; Sixteenth Annual Report, parti (1896), figs. 89, 
96, 97. A careful study of these microscopic drawings and photographs will show 
that in all cases these minute faults along which more or less slippage has occurred 
are the necessary result of the lateral shortening of the rock mass by plication. This 
plication was as much due to a lateral compression as was the major folding of the 
Appalachian system. Slip cleavage should therefore not be confounded with two 
intersecting systems of fractures (jointing on a small scale), the strike of neither of 
which would be at right angles to the direction of strain but diagonal to it or to a sin- 
gle system of such fractures which might be the result of stretching. The same pres- 
sure which produced slip cleavage in buried masses of micaceous matter (slate and 
schist) may have produced fractures at the surface; but where both schist and a rigid, 
vitreous rock like quartzite have (in contact) been subjected to the same pressure, in 
the Taconic region, both have usually been folded, the schist in smaller folds, the 
quartzite in larger ones, but slip cleavage has been confined to the schist, as shown 
in fig. 25 of Thirteenth Annual Report, already referred to. 
The faults or fractures in slip cleavage belong strictly to the process of "rock 
flowage" and not to that of "rock fracture." As 1 [eim put it: " By real cleavage is 
always to be understood a cleavability of the material pervading the entire mass and 
visible in each little fragment. ... . " a 
THE GRAIN. 
Sharpe's explanation for slates splitting more readily along the "grain" than 
across it is that the mineral particles lie with their flat surfaces parallel to the cleavage 
and their longer axes in the direction of the cleavage dip. A fracture across the 
cleavage and parallel to the dip is parallel to the longer sides of the particles, whereas 
one parallel to the strike of the cleavage is across both longer axes and sides. & 
" Mechanismus der Gebirgsbildung, vol. 2, p. 59. 
&Op. cit., p. 114. 
