112 



STRUCTURAL GEOLOGY. 



waters from time to time tumbles down an outer range, and exposes a new 

 series of structures. 



Traversing the surface of a region thus intersected, the joints appear as 

 mere fractures, and are remarkable mainly for their great extent, number, 





121. 



Jointed rocks, Cayuga Lake. Hall, '43. 



122 



and uniformity. In case of two systems of joints, — the case most common, 

 — the rock breaks into blocks, which are rectangular or rhomboidal, accord- 

 ing as the joints cross at right angles or not. The main system of joints is 

 sometimes parallel to the strike of the uplifts, or else to the range of eleva- 

 tions or mountains in the vicinity, or to some general mountain range of the 

 continent. 



In man}^ 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. 122. This case occurs in a section of 

 part of a quartzyte bluff on the railroad near 

 Poughquag, Dutchess County, N.Y. a, a, a 

 are ordinary joints in the stratified rock; h, 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 intersecting dike, or of a portion of the laminated sandstone 

 set erect. It occurs in sand-beds, whose grains adjust easily, like shot, to 

 pressure. 



Fig. 124 represents a rock with two cleavage-directions ; and 125 a quartz- 

 ose sandstone which has irregular cleavage-lines. These last two cases, 

 together with that represented in Fig. 122, appear to show that the jointed 

 structure and slaty cleavage may have a similar origin. 



Slaty and foliated structure. — In the slaty structure, or slaty cleavage, 

 the rock is divided into thin even sheets or laminae, as in the case of roofing- 

 slate or writing-slate. The laminated structure of shales is parallel to the 

 bedding, and is due to the conditions of deposition and the pressure of super- 



Jointed quartzyte. D. '72. 



