THEORIES OF SLATE. 79 



In certain cases, however, he observed that the bedding was almost 

 obliterated by the disturbances due to the pressure. The supplementary 

 hypothesis is that the preliminary effect of pressure is to give the mica 

 an irregular distribution, the final effect being to rearrange the mica 

 scales in the planes of cleavage.* 



Objections to the Hypothesis of Heterogeneity — In my opinion, there are 

 the gravest objections to the hypothesis that slaty cleavage is due to the 

 lack of homogeneity of a rock mass which has been subjected to the 

 action of force. Neither Tyndall nor Daubree found that the presence 

 of scales promoted schistosity, but just the reverse. The wax employed 

 by Tyndall may have consisted largely of prismatic bodies ; but before 

 pressing his wax he softened it, making these bodies, as well as their 

 groundmass, very plastic. He also kneaded the mass, so that the com- 

 ponent particles must have welded. Even if every one of the prisms had 

 assumed a horizontal position, there is no reason to suppose that the 

 cohesion between them and the groundmass of the wax was feebler than 

 that between the different portions of any one prism, or that any schis- 

 tosity, at all approaching slaty cleavage, would have resulted. Similar 

 remarks apply to Daubree's experiments on clay. 



Dr Sorby's supplementary hypothesis is suggestive in the same con- 

 nection. All geologists will grant that disturbances are sometimes such 

 as nearly or quite to obliterate the bedding of shales, but none will assert 

 that this is a condition of slaty cleavage. We all know that the bedding 

 is often most distinctly preserved in masses of roofing slate, and that the 

 lamination is not infrequently fairly regular. In such cases it seems to 

 me impossible to contend that the mica scales originally concordant with 

 the bedding have been stirred up in such a manner as to be distributed 

 at all angles through the mass. Again, there are many somewhat indu- 

 rated shales not affected by slaty cleavage in which there are countless 

 mica scales, nearly all of them concordant with the bedding. If the dis- 

 tribution of mica scales constituted the fissility called slaty cleavage, 

 such beds should split like slate in the planes of bedding. Such beds 

 are sometimes fissile to a certain extent, but cases in which this fissility 

 could be mistaken for slaty cleavage are very rare, if, indeed, any are 

 known. When rocks split along their lamination at all like slate, geolo- 

 gists expect to find, and usually do find, that the rock possesses true slaty 

 cleavage coinciding locally in direction with the planes of bedding, but 

 superinduced upon and independent of bedding; 



Similar objections apply to Mr Sharpe's theory of the flattening of the 

 rock components. It affords no explanation of Professor Tyndall's ex- 

 periments, and were it correct some tine-grained sandstones, at any rate, 



*Q. Joui - . Gcol. Soc, vol. xxxvi, 1880, p. 7;i. 

 XII— Bull. Geol. Soc. Am., Vol. 4, 1892. 



