Ch. XXX.1 TESTS OF AGE OF VOLCANIC BOCKS. 519 



CHAPTER XXX. 



ON THE DIFFERENT AGES OP THE VOLCANIC ROCKS. 



Tests of relative ages of volcanic rocks — Tests by superposition and intrusion — 

 Dike of Quarrington Hill, Durham — Test by alteration of rocks in contact — 

 Test by organic remains — Test of age by mineral character — Test by included 

 fragments — Volcanic rocks of the Post-Pliocene period — Basalt of Bay of 

 Trezza in Sicily — Post-Pliocene volcanic rocks near Naples — Dikes of Somma 

 — Igneous formations of the Newer Pliocene period — Val di Noto in Sicily. 



Haying referred the sedimentary strata to a long succession of geo- 

 logical periods, we have now to consider how far the volcanic formations 

 can he classed in a similar chronological order. The tests of relative 

 age in this class of rocks are four : — 1st, superposition and intrusion, 

 with or without alteration of the rocks in contact ; 2d, organic remains ; 

 3d, mineral characters ; 4th, included fragments of older rocks. 



Tests by superposition, &c. — If a volcanic rock rests upon an aqueous 

 deposit, the former must be the newest of the twCj hut the like rule does 

 not hold good where the aqueous formation rests upon the volcanic, for 

 melted matter, rising from below, may penetrate a sedimentary mass 

 without reaching the surface, or may be forced in conformably between 

 two strata, as 6 at D in the annexed figure (fig. 656), after which it may 

 cool down and consolidate. Superposition, therefore, is not of the same 



value as a test of age in the unstratified volcanic rocks as in fossiliferous 

 formations. We can only rely implicitly on this test where the volcanic 

 rocks are contemporaneous, not where they are intrusive. Now they 

 are said to be contemporaneous if produced by volcanic action, which 

 was going on simultaneously with the deposition of the strata with which 

 they are associated. Thus in the section at D (fig. 656), we may per- 

 haps ascertain that the trap b flowed over the fossiliferous bed c, and 

 that, after its consolidation, a was deposited upon it, a and c both belong- 

 ing to the same geological period. But if the stratum a be altered by 

 b at the point of contact, we must then conclude the trap to have been 

 intrusive, or if, in pursuing b for some distance, we find at length that 

 it cuts through the stratum a, and then overlies it as at E. 



We may, however, be easily deceived in supposing a volcanic rook to 

 be intrusive, when in reality it is contemporaneous ; for a sheet of lava, 

 as it spreads over the bottom of the sea, cannot rest every, where- upon 

 the same stratum, either because these have been denuded,. or because, 



