Ch. XXV.] RELATIONS OF GRANITE AND TRAP. 361 



igneous rocks of Scotland, observes ' that it is a mere dispute 

 about terms to refuse to the ancient eruptions of trap the name 

 of submarine volcanos, for they are such in every essential 

 point, although they no longer eject fire and smoke*/ 



The same author also considers it not improbable that some 

 of the volcanic rocks of the same country may have been 

 poured out in the open airf . 



The recent examination of the igneous rocks of Sicily, 

 especially those of the Val di Noto, has proved that all the 

 more ordinary varieties of European trap have been produced 

 under the waters of the sea in the Newer Pliocene period, that 

 is to say, since the Mediterranean has been inhabited by a great 

 proportion of the existing species of testacea. We are, there- 

 fore, entitled to feel the utmost confidence, that if we could 

 obtain access to the existing bed of the ocean, and explore the 

 igneous rocks poured out within the last 5000 years beneath 

 the pressure of a sea of considerable depth, we should behold 

 formations of modern date scarcely distinguishable from the 

 most ancient trap rocks of our island. We cannot, however, 

 expect the identity to be perfect, for time is ever working 

 some alteration in the composition of these mineral masses, as, 

 for example, by converting porous lava into amygdaloids. 



Passage from trap into granite. If a division be attempted 

 between the trappean and volcanic rocks, it must be made 

 between different parts of the same volcano, nay even the same 

 rock, which would be called ' trap/ where it fills a fissure and 

 has assumed a solid crystalline form on slow cooling, must be 

 termed volcanic, or lava, where it issues on the flanks of the 

 mountain. Some geologists may perhaps be of opinion that 

 melted matter, which has been poured out in the open air, 

 may be conveniently called volcanic, while that which appears 

 to have cooled at the bottom of the sea, or under pressure, but 

 at no great depth from the surface, may be termed f trap ;' 

 but we believe that such distinctions will lead only to con- 

 fusion, and that we must consider trap and volcanic as syno- 

 nymous. On the other hand, the difficulty of discrimi- 

 * Syst, of Geol., vol. ii. p. 1 14. f Ibid^ 



