PART II. CHAPTER XIII. 271 



Tests of Relative Age of Volcanic Rocks. 



CHAPTER XXIII. 



ON THE DIFFERENT AGES OF THE VOLCANIC ROCKS. 



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

 By alteration of rocks in contact Test by organic remains Test of age by 

 mineral character Test by included fragments Volcanic rocks of the Recent 

 and Newer Pliocene periods Miocene Eocene Cretaceous Oolitic New 

 Red sandstone period Carboniferous Old Red sandstone period Silurian 

 Upper and lower Cambrian periods Relative ages of intrusive traps. 



HAVING referred the sedimentary strata to a long succession 

 of geological periods, we have next to consider how far the vol- 

 canic formations 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 character ; 

 4th, included fragments of older rocks. 



Test by superposition, fyc. If a volcanic rock rest upon an 

 aqueous deposit, the former must be the newest of the two, but 

 the like rule does not hold good where the aqueous formation 

 rests upon the volcanic, for we have already seen (p. 110.) that 

 melted matter, rising from below, may penetrate a sedimentary 

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

 formably between two strata, as b at D in the annexed figure 

 (Fig. 284.) after which it may cool down and consolidate. Su- 



Fig. 284. 



perposition, 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 sec- 



