76 G. F. BECKER — FINITE STRAIN IN ROCKS. 



in one direction, which appears to be usually that of the dip of the slate 

 in its original position. A block of slate thus bears some analogy to a 

 block of wood, so far as its fissility is concerned. In some cases, how- 

 ever, slates are said to split equally well from any edge of a block. 

 Fossils occurring in slate are usually distorted, and numerous measure- 

 ments of these fossils have been made for the purpose of ascertaining in 

 which direction the greatest elongation and contraction have taken place. 

 As might be expected, these measurements do not accord very closely, 

 for it is difficult to expose a fossil in such a manner that all its dimen- 

 sions are accessible without obscuring the relations of these dimensions 

 to the dip and strike of the slate. Sometimes there seems to be no rela- 

 tive distortion in the plane of the cleavage. In other cases the fossils 

 are greatly distorted in the cleavage plane, the longer axis coinciding 

 with the dip. 



Figure 17. — Steps in Slate. 



It is frequently asserted that the greatest elongation of the fossils is 

 always in the direction of the grain of the slate, and the greatest con- 

 traction perpendicular to this direction. This implies that there has 

 been no tangential movement among the laminae, or that there is no 

 fluxion structure and no close jointing or "Ausweichungsclivage " in the 

 rock; for in any such case tlie axes of the strain ellipsoid must fail to 

 coincide with the dip, the strike, and the perpendicular to the cleavage. 

 Now these structures are known to be frequent in slaty rocks and dis- 

 tinguisbable from true, slaty cleavage only under the microscope. The 

 deductions from the measurements of the fossils can therefore be only 

 approximately true. I have myself seen fossils in slate in which fluxion 

 structure was plainly manifested, in my opinion, and in tbat of an emi- 

 nent paleontologist whom I consulted. Slaty developments of crystal- 

 line rocks are by no means unknown, and these are closely allied to 



