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 air jr. 
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 c trap, 1 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 c 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. 114. f Ibid- 
