101 



These are masses far more extensive in two 

 dimensions than in the third, and commonly oc- 

 cupying large spaces. In its simplest form, the 

 stratum may be considered as a horizontal bed, 

 the superior and inferior planes of which are pa- 

 rallel, or nearly so, and straight, or lying in one 

 mathematical plane. It is held to arise from the 

 deposition of earthy matters from water. That 

 form is however rare, and the stratum is subject 

 to many modifications. 



The planes may be straight, and inclined to 

 each other ; so that the stratum, if prolonged, 

 would terminate in an edge. 



By the gradual approximation of the planes 

 and their successive divergence, they may vary in 

 thickness in different parts ; and, in such cases, 

 it is obvious that the planes will no longer be ma- 

 thematically straight, but variously curved. 



In these cases, strata sometimes vanish en- 

 tirely in the planes of their courses, and are re- 

 newed at some distance, with the same or with 

 other connections. 



Whether parallel, or not, they vary extremely 

 in thickness ; in some cases being even less than 



