62 J. Tf. Jiidd — On Yolcanos. 



where alteration has clearly been produced in the sedimentary rocks 

 in the vicinity of the igneous ; then in all these cases the latter are 

 undoubtedly of more recent date than the former. 



But where, on the other hand, the fossiliferous sedimentary rocks 

 overlie the whole of the igneous rocks occupying the depressions 

 produced in them by denudation, or where the former contain pebbles 

 derived from the latter, then we may confidently pronounce the 

 igneous masses to be the older of the two. 



It is evident that if the relations of a series of igneous rocks 

 be thus studied, we may often, by excluding certain geological 

 periods as certainly more recent, and others as undoubtedly more 

 ancient, arrive at an approximate date for the outbursts by which 

 they were formed. 



If, however, the igneous masses, whether lava-streams or tuffs, are 

 seen to be interbedded with a series of fossiliferous sediments, the 

 former having clearly produced alteration in the underlying strata, 

 while those superposed to them are unaffected, — then we are perfectly 

 safe in inferring that both the igneous and aqueous rocks were pro- 

 duced during the same geological period. 



There are undoubtedly cases in which we are unable to apply 

 either of the methods which we have noticed above, whether direct 

 or inferential, to the determination of the age of a series of volcanic 

 deposits. In such cases, however, the analogies or relations of such 

 io-neous masses with others of which the date can be determined, 

 may be sufficient to warrant us in assuming with a high degree of 

 probability that they were formed during the same period. 



Some geologists have maintained the opinion that the chemical 

 and mineralogical features of volcanic rocks afford in themselves 

 sufficient evidence of the date of their formation. But we cannot 

 admit, in the present condition of our knowledge, such to be the 

 case ; there are certainly rocks of the most widely different age, but 

 which, having been subjected to similar processes of alteration, are 

 now quite undistinguishable in character by any test which we are 

 able to apply. We shall not, however, be misunderstood as uphold- 

 ing the converse of this position, and asserting that all igneous rocks 

 of former geological periods are identical in character and compo- 

 sition with those being formed at the present day. On the contrary, 

 we are driven to the conclusion that while many facts seem to point 

 to the identity of character of ancient and modern igneous rocks, 

 there are not a few others which may be cited as tending to prove 

 that, alike in their ultimate composition and their component mine- 

 rals, the igneous rocks exhibit remarkable peculiarities in their 

 distribution, both in space and time. 



As the result of the studies carried on by geologists concerning 

 the age and relations of the igneous rocks in different districts, no 

 fact comes out more strikingly than that of the constant shifting of 

 the centres of volcanic action. We find unmistakable evidence of 

 the alternation in the same area of epochs characterized by great 

 volcanic outbursts, with others in which the gradual deposition of 

 sediments was clearly not interfered with by any such cause. 



