GENERAL LAWS IN GEOLOGY. 591 



tions from one to another, by the effect of an 

 interior developement" 



The phenomena of alternation to which M. de 

 Humboldt here refers are, in fact, very curious : as 

 exhibiting a mode in which the transitions from one 

 formation to another may become gradual and in- 

 sensible, instead of sudden and abrupt. Thus the 

 coal measures in the south of England are above 

 the mountain limestone ; and the distinction of the 

 formations is of the most marked kind. But as we 

 advance northward into the coal-field of Yorkshire 

 and Durham, the subjacent limestone begins to be 

 subdivided by thick masses of sandstone and carbo- 

 naceous strata, and passes into a complex deposit, 

 not distinguishable from the overlying coal mea- 

 sures ; and in this manner the transition from the 

 limestone to the coal is made by alternation. Thus, 

 to use another expression of M. de Humboldt's, in 

 ascending from the limestone, the coal, before we 

 quit the subjacent stratum, preludes to its fuller 

 exhibition in the superior beds. 



Again, as to another point: geologists have gone 

 on up to the present time endeavouring to dis- 

 cover general laws and facts, with regard to the 

 position of mountain and mineral masses upon the 

 surface of the earth. Thus M. Von Buch, in his phy- 

 sical description of the Canaries, has given a mas- 

 terly description of the lines of volcanic action and 

 volcanic products, all over the globe. And, more 

 recently, M. Elie de Beaumont has offered some 

 generalizations of a still wider kind. In this new 



