438 CERORGE EP BECKER 
possible to break out masses with very acute rhombic cross-sec- 
tions which in the case illustrated in Fig. 3 would have angles of 
13°, but such rhombs would themselves be cleavable into still 
more acute flakes. I should call such a mass a schist (whether 
crystalline or not), reserving the name s/ate for a more regular 
structure.t Sir Archibald Geikie distinguishes slate from schists 
calling slates ‘‘cleaved” and schists “foliated;” he makes 
approximately parallel lenticular and usually wavy layers or folia 
characteristic of the schists.2 This definition seems to answer 
to my use of the term and to the explanation given above, but 
many geologists use the terms slate and schist almost inter- 
changeably. In the interest of precision it is most desirable 
that slate and schist should be distinguished and that geologists 
“should define the sense in which they employ these terms. 
Passing now to the second strain to be discussed it will be 
‘well to state how scission is produced. The forces acting on any 
‘small cubical portion of a strained mass are reducible to three 
‘forces which are normal to faces of the cube, and a couple. If 
ithe mass is in equilibrium these forces and this couple are exactly 
balanced by the resistances which the mass itself opposes to 
strain. The normal forces produce changes of volume and pure 
shears such as have been discussed above. The effect of the 
couple is to produce the distortion here called a scission and 
more usually known as a “‘simple shear” or a “‘shearing motion,” 
but never as pure shear. The origin of scission is thus differ- 
ent from that of pure shear. Because it arises from the action 
of a couple, it is called a rotational strain. Scission is quite as 
important as pure shear. It may be said to be present in practi- 
cally all real strains because the absence of scission characterizes 
only a limiting case which is only approximately realizable even 
with refined apparatus. Scission is not a strain which by itself 
There seems no logic in the constant employment of the term “crystalline” 
schists unless these are to be distinguished from other schists not crystalline. 
2 Text-book of Geol., 1893, p. 103. 
3 The pole about which the couple tends to produce rotation does not in general 
coincide with the direction of the maximum, minimum or mean normal stresses. One 
