SCHISTOSIM VAN DD, SALTY: CLEAVAGE: 
SOME years since I published a paper on the finite, homoge- 
neous strain, flow and rupture of rocks‘ which contained with 
other matter a new theory of slaty cleavage and the allied less 
regular schistose cleavage. Colleagues have lately informed me 
that this paper is too mathematical for their convenience, and | 
therefore propose to discuss schistosity and cleavage without 
mathematics. The result cannot be wholly satisfactory in the 
nature of the case; nevertheless this presentation will suffice 
for those who care little about the matter and will make my 
former discussion easier to those who are more interested. No 
part of that discussion is really difficult, but the chain of rea- 
soning is unavoidably long and therefore trying to the patience. 
The general idea to be developed is that the deformation of a 
solid, homogeneous, viscous, isotropic, not infinitely brittle mass 
will develop structure in it, on not less than one surface nor on 
more than four surfaces simultaneously. These structure sur- 
faces will in general stand at acute angles to the direction of 
the pressure to which they are due and the flattening of the 
strain ellipsoids will not be normal to the pressure except ina 
limiting case. The assumptions needful to prove these proposi- 
tions are almost axiomatic, viz., stresses and strains are of the 
same order of magnitude ; a solid mass opposes deformation by 
forces which are divisible into those independent of the time 
rate of straining and those which are dependent on this rate. 
*Geological Soc. of Amer., Vol. IV., 1893, pp. 13-90. An earlier paper, the 
structure of a portion of the Sierra Nevada of California, appeared in the same series, 
Vol. IL, 1891, p.49, and a later one, the finite elastic stress strain function, was printed 
in the Amer Jour. of Science, Vol. XLVI., 1893, p. 337. A paper on the torsional 
theory of pomts may be found in Trans. Amer. Inst. Mining Engineers, Vol. XXIV., 
1894, p 130. I may mention in the same connection an essay on distributed faults, 
U.S. Geol. Sur., Monograph III., 1882, chap. iv. 
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