SLATY STRUCTURE. 



89 



A. 



Traversing the surface of a region thus intersected, the joints ap- 

 pear as mere fractures, and are remarkable mainly for their great 

 extent, number and uniformity. In case of two systems of joints, — 

 the case most common, — the rock breaks into blocks, which are rect- 

 angular or rhomboidal, according as the joints cross at right angles or 

 not. The main system of joints is usually parallel to the strike of 

 the uplifts, or else to the range of elevations or mountains in the 

 vicinity, or to some general mountain-range of the continent ; and the 

 directions are studied with much interest, because of their bearing 

 upon the geological history of the country. 



In many cases, a rock is so evenly and extensively jointed as to 

 become thereby laminated, and in such a case the joints may be easily 

 mistaken for planes of stratification, especially when the latter have 

 been obliterated. Sometimes there are 

 sudden transitions from the regular 

 stratification to vertical joints, as in Fig. 

 88 A. This case occurs in a section of 

 part of a quartzyte bluff on the railroad 

 near Poughquag, Dutchess Co., N. Y. 

 a, a, a, are ordinary joints in the strati- 

 fied rock ; b, b, is a portion of the rock, 

 which has lost its stratification entirely, 

 and has become jointed vertically ; the transition from the stratified to 

 the part b, b, is so abrupt that the latter has the aspect of an inter- 

 secting dike, or of a portion of the laminated sandstone set erect. 



c. The Slaty Structure. — The slaty structure or slaty cleavage, as 

 it is called — is in some cases parallel with the planes of deposition 

 or bedding of a rock ; and such examples of it come under a former 

 head. But in many of the great slate regions, as in that of Wales, 

 the slate lamination is transverse to the bedding, as shown in 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 Columbia 

 County, N. Y. 



Occasionally, the lines of deposition are in- 

 dicated by a slight flexure in the slate near 

 them, as in Fig. 90. In other cases, there 

 is a thin intermediate layer 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 represents a rock with two cleavage-directions; and 



Fig. 89. 



