SLATY STRUCTURE. 



101 



fig. 89, in which the lines a, b, c, d show the lines of bedding, and 

 the oblique lines the direction of the slates. Whole mountains 

 have sometimes this kind of oblique or transverse lamination. 



The sketch, fig. 89, by Mather, 

 is from the slate region of Colum- 



Fig. 89. 



bia county, JS\Y. 



Occasionally the lines of deposition are 

 indicated by a slight flexure in the slates 

 at the spot, ae in fig. 90. In other cases 

 there is a thin intermediate layer of quartz 

 rock or limestone which does not partake 



of the cleavage. Fig. 91 represents an interstratification of clay-layers with 

 limestone, in which the former have the cleavage, but not the latter, — though the 

 limestone sometimes shows a tendency to it when argillaceous. Fig. 92 repre- 



Fig. 91. 



sents a rock with two cleavage-directions ; and 93 a quartzose sandstone which 

 has irregular cleavage-lines. These last two cases show that the jointed structure 

 is but one variety of the cleavage-structure, and that both have the same origin. 



Fi S- 92 ' Fig. 93. 



Sedgwick first detected the true lines of bedding, and ascertained that the 

 slaty structure was one that had been superinduced upon the clayey strata by 

 some process carried on since they were first deposited. 



The foliated structure (or foliation) of mica schist, gneiss, and related schist- 

 ose rocks appears to be sometimes transverse to the bedding, like most slaty 

 cleavage. But, as in the slates, it is not universally so, and the rock in each 

 region requires a special examination with reference to this point. 



3. Positions of Strata. 

 110. The natural positions of strata as formed, and the positions 

 resulting from the disturbance or dislocations of strata, are two dis- 

 tinct topics for consideration in this place. 



