36 DETERMINATION OF THE [Ch. IV. 



masses has been disturbed, at different periods, by convulsions 

 of extraordinary violence, as in the Alps and other mountain- 

 ous districts, there are instances where the original position of 

 strata has been reversed ; but such exceptions are rare, and are 

 usually on a small scale, and an experienced observer can 

 generally ascertain the true relations of the rocks in question, 

 by examining some adjoining districts where the derangement 

 has been less extensive. 



In regard to volcanic formations, if we find a stratum of tuff 

 or ejected matter, or a stream of lava covering sedimentary 

 strata, we may infer, with confidence, that the igneous rock is 

 the more recent ; but, on the other hand, the superposition of 

 aqueous deposits to a volcanic mass does not always prove the 

 former to be of newer origin. If, indeed, we discover strata of 

 tuff with imbedded shells, or, as in the Vicentine and other 

 places, rolled blocks of lava with adhering shells and corals, 

 we may then be sure that these masses of volcanic origin covered 

 the bottom of the sea, before the superincumbent strata were 

 thrown down. But as lava rises from below, and does not 

 always reach the surface, it may sometimes penetrate a certain 

 number of strata, and then cool down, so as to constitute a 

 solid mass of newer origin, although inferior in position. It is, 

 for the most part, by the passage of veins proceeding from 

 such igneous rocks through contiguous sedimentary strata, or 

 by such hardening and other alteration of the overlying bed, as 

 might be expected to result from contact with a heated mass, 

 that we are enabled to decide whether the volcanic matter was 

 previously consolidated, or subsequently introduced. 



Proofs by included fragments of older rocks. 

 A geologist is sometimes at a loss, after investigating a dis- 

 trict composed of two distinct formations, to determine the 

 relative ages of each, from want of sections exhibiting their 

 superposition. In such cases, another kind of evidence, of a 

 character no less conclusive, can sometimes be obtained. One 

 group of strata has frequently been derived from the degrada- 



