100 LITHOLOGICAL GEOLOGY. 



through great depths. The planes of division are often as even as 

 if a thin blade had been drawn through with a clean long stroke. 

 These joints may be in one, two. or more directions in the same 

 rock, and they often extend, with nearly uniform directions, through 

 regions that are hundreds of miles in length or breadth. The ac- 



Fig. 88. 



companying sketch represents the falling cliffs of Cayuga Lake, 

 and the fortress-shapes and buttresses arising from the natural 

 joints intersecting the rocks. The wear of the waters from time 

 to time tumbles down an old surface and exposes a new range 

 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, 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. In some places a layer looks like a rectangular pavement on 

 a vast scale. In others, where the layers are thick and coarse and 

 somewhat displaced, there is a resemblance to artificial fortifica- 

 tions, or cities in ruins, which is quite striking. 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. 



The joints in rocks, when not too numerous, are often a great 

 assistance to quarrymen in quarrying rock, as they afford natural 

 sections of the layers. 



109. c. Cleavage, or the Slaty Structure. — The slaty structure — or 

 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 



