Ch. XXX.] RELATIVE AGE OF VOLCANIC ROCKS. 655 



CHAPTER XXX. 



OX THE DIFFERENT AGES OF THE VOLCANIC ROCKS. 



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

 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 the Bay of Trezza in Sicily — Post-Pliocene volcanic 

 rocks near Naples — Dikes of Somma. 



Having referred the sedimentary strata to a long succession of 

 geological periods, we have now to consider how far the volcanic for 

 mations can be 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, etc. — If a volcanic rock rest upon an aque- 

 ous deposit, the former must he the newest of the two ; but the like 

 rule does not hold good where the aqueous formation rests upon the 

 volcanic, for melted matter, rising from below, may penetrate a sedi- 

 mentary mass without reaching the surface, or may be forced in con- 

 formably between two strata, as b at d in the annexed figure (fig. 

 710), after which it may cool down and consolidate. Superposition, 



Fig. 710. 



E 





D 





\ b , ' \ ' ■, 1 



7C>?-«c: — 



a 



-, J 



^^jvZZ^I 



>i ',',' /* ' .' l'l 



1 . ' ' | 



c 





C 







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 im- 

 plicitly on this test where the volcanic rocks are contemporaneous, 

 not where they are intrusive. Xow, they are said to be contempora- 

 neous if produced by volcanic action which was going on simultane- 

 ously with the deposition of the strata with which they are asso- 

 ciated. Thus in the section at d (fig.. 710), we may perhaps 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 belonging 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 intru- 



