TERiiANES. 



113 



incumbent beds ; but slates have received their structure from lateral pres- 

 sure, and it often crosses the bedding, as in Figs. 126, 127. This structure is 



123. 



Jointed rocks. De la Beche. 



126. 



also called the foliated structure. The sections represented in Figs. 126, 127 

 are from the slate region of Columbia County, N.Y. 



Occasionally, the lines of deposition are indicated by a slight flexure in 

 the slate near them, as in Fig. 127. In other cases there is a thin intermedi- 

 ate layer which does not 

 partake of the cleavage. 

 Fig. 123 represents an in- 

 terstratification of clay- 

 layers with limestone, in 

 which the former have 

 the cleavage, but the lat- 

 ter not, though the lime- 

 stone sometimes shows a tendency to it where argillaceous. 



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 since they were first deposited. 



The schistose structure of crystalline rocks, or their schistosity, as it is 

 often termed, may be produced by pressure ; and hence all schistose struc- 

 tiire, and even the fainter parallelism of the planes of a foliated mineral like 

 mica, as in granitoid gneiss, are often termed foliated. The regular fractures 

 producing a jointed or a slaty structure are named diadases by Daubr^e, 

 and fractures accompanied by displacement, paradases. 



Slaty cleavage, Columbia Co., N.Y. Mather, '43. 



4. Calculating the Thickness of Strata. 



When strata are inclined, as in Fig. 128, the thickness is ascertained by 

 measuring the extent along a horizontal surface, and also the angle of dip, 

 and then calculating the thickness by trigonometry. The thickness of the 

 strata from a to b is bd, the line bd being drawn at right angles to the 

 strata. Measuring ab and the dip, which is the angle bad, the angles and 

 hypotenuse of the triangle abd are given to determine one side bd. Or, 

 with the distance ae, the side ce would be found. 



But for correct results, the absence of faults must be first ascertained. 



DANAS MANUAL 



