G. F. Becker — Current Theories of Slaty Cleavage. 9 



broken out of the metamorphosed mass would bring away a 

 " beard " of schist standing at an angle to its axis. 



These specific instances merely illustrate the general conclu- 

 sion that the elongation of pebbles gives no information of any 

 value as to the position of the strain ellipsoid. The same con- 

 clusion is immediately applicable to the flattening of angular 

 mineral fragments. 



The evidence afforded by fossils is as a rule no better than 

 that derived from pebbles. It is easy to imagine an ellipsoid 

 circumscribed about a trilobite for example, and then infer the 

 apparent distortion in a given strain. The discordant results 

 which have been deduced from the measurements of fossils by 

 various observers are thus easily accounted for.* Nevertheless, 

 with a sufficient amount of work I believe better results could 

 be obtained. In a strained fossil there must be two central 

 sections which are undistorted and possibly these might be 

 found by accurate measurements in some favorable cases. If 

 found, they would determine the position and the ellipticity 

 of the section of the ellipsoid through the greatest and least 

 axes. To determine with accuracy the undistorted sections of 

 a fossil would be a delicate job and has not been attempted so 

 far as I know. 



The evidence from volcanic textures, such as the blow holes 

 in pre-Cambrian lavas, is as poor as that from pebbles, since 

 blow holes are substantially always ellipsoidal in unmetamor- 

 phosed lavas. 



Mr. Leith also attempts to use the distortion of beds and the 

 attitude of folds to prove the position of the minor axes of the 

 strain ellipsoid. In certain cases beds of a composition some- 

 what different from that of the slate are crenulated in a direc- 

 tion normal to the cleavage, and Mr. Leith considers this a 

 proof that the " greatest shortening " of the mass is also normal 

 to the cleavage. In figure 4, which illustrates my own theory 

 of cleavage, I have drawn a bed before and after distortion, 

 assuming that this bed retains its original length unchanged 

 because of lack of plasticity. The axis of crenulation is exactly 



* A part of the difficulty seems to be due to the fact that even flat organ- 

 isms are by no means always deposited in strict conformity to the stratifica- 

 tion. This is apparent in recent muds and in unaltered sedimentary rocks. 

 Among schistose rocks a good example is afforded by the Ordovician slates 

 of Arvonia, Virginia. They contain Cyclocystoides which were originally 

 circular flattened discs and have been converted into very regular ellipses 

 with axes in the average proportion of 3 to 2 or thereabout. I have examined 

 some specimens of this slate a few square inches in area on which the orienta- 

 tion of the little ellipses varied by at least 45° and in no regular manner, 

 doubtless because of original variation in position. Again, where the plane 

 of cleavage approaches the plane of sedimentation, but does not coincide 

 with it, fossils may undergo a deceptive distortion. Thus in the Arvonia 

 slates there are cases where the ellipses representing Cyclostoides are very 

 regularly oriented, but with their major axes at an angle of some 20° to the 

 grain of the slate. 



