GEOLOGY. 



" A seam is a thin layer of rock that separates the beds or 

 strata of another rock, as a seam of coal, limestone, <fec." 



A joint is a separation of rocks, both stratified and unstrati- 

 fied, into masses of some determinate shape : joints are more 

 or less parallel, and usually cross the beds obliquely. 



Cleavage planes are divisions in rocks, which do not coincide 

 with those of stratification, lamination or joints. They are 

 supposed to result from a crystaline arrangement of the par- 

 ticles of the rock. 



Fig 2 Cleavage Planes. 



d A A 



A A a B 



[Fig. 2 exhibits the planes of stratification, B, B, the joints, A, A, A, 

 A, and the slaty cleavage, d, d.] 



Horizontal strata are those which have little or no inclina - 

 tion, but lie parallel Fig " 3 ' Horizontal *** 



with the horizon: this 

 position, however, 

 rare, almost all strata being more or less inclined. 



The Dip of strata signifies the angle which they form with 



the horizon. Fig. 4. Dip and Outcrop. 



utcrop. When 

 strata are uncovered 

 above the surface, or 

 protrude from the side of a hill so as to be visible, they are 

 said to crop out. 



An Escarpment is formed when strata terminate abruptly, 

 so as to form a precipice. 



