272 



LYELL'S ELEMENTS OF GEOLOGY. 



Tests of Relative Age of Volcanic Rocks 



tion at D (Fig. 284.), w r e may perhaps ascertain that the trap b 

 flowed over the fossiliierous bed c, and that, after its consolida- 

 tion, 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 

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

 that it cuts through the stratum a, and then overlies it. 



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

 rock 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, if newly thrown down, they thin 

 out in certain places, thus allowing the lava to cross their edges. 

 Besides, the heavy igneous fluid will often, as it moves along, 

 cut a channel into beds of soft mud and sand. Suppose the sub- 

 marine lava F, to have come in contact in this manner with the 



strata a, &, c, and that, after its 

 Fig. 285. consolidation, the strata d, e, are 



thrown down in a nearly horizon- 

 tal position, yet so as to lie uncon- 

 formably to F, the appearance of 

 subsequent intrusion will here be 

 complete, although the trap is in 

 fact contemporaneous. We must, 

 unless we find the strata d or e to 

 have been altered at thoir junction, as if by heat, not therefore 

 hastily infer that the rock F is intrusive. 



When trap dikes were described in the 8th chapter, they were 

 shown to be more modern than all the strata which they tra- 

 verse. The ninety-fathom dike in the Northumberland coal- 

 field (see section, Fig. 286.) passes through coal-measures which 



Magnesian limestone. 



Coal. 



Dike. 

 Section in a quarry at Cullercoats, Northumberland. 



are much disturbed.* The beds of overlying Magnesian lime- 

 stone are not cut through by the dike, but appear to be in the 



* See Mr. Winch's account, Geol. Trans. 1st ser. vol. iv. p. 1. 



