488 "RELATION OF TRAP, [Cn. XXIX 



These igneous rocks of the Val di Noto, and the more ancient ti'appeac 

 rocks of Scothmd and other countries, difier from subaerial volcanic for- 

 mations in being more compact and heavy, and in forming sometimes 

 extensive sheets of matter intercalated between marine strata, and some- 

 times stratified conglomerates, of which the rounded pebbles aie all trap. 

 They differ also in the absence of regular cones and craters, and in the 

 want of conformity of the lava to the lowest levels of existing valleys. 



It is highly probable, however, that insular cones did exist in some 

 parts of the Val di Noto ; and that they were removed by the waves, 

 in the same manner as the cone of Graham Island, in the Mediterra- 

 nean, was swept away in 1831, and that of Nyoe, off Iceland, in 

 1783.^'' All that would remain in such cases, after the bed of the 

 sea has been upheaved and laid dry, would be dikes and shapeless 

 masses of igneous rock, cutting through sheets of lava which may have 

 spread over the level bottom of the sea, and strata of tuff, formed of ma- 

 terials first scattered far and wide by the winds and waves, and then de- 

 posited. Conglomerates also, with pebbles of trap, to which the action 

 of the waves nmst give rise during the denudation of such volcanic 

 islands, wijl emerge from the deep whenever the bottom of the sea be- 

 comes land. The proportion of volcanic matter which is originally sub- 

 marine must always be very great, as those volcanic vents which are not 

 entirely beneath the sea are almost all of them in islands, or, if on conti- 

 nents, near the shore. 



As to the absence of porosity in the trappean formations, the appear- 

 ances are in a great degree deceptive, for all amygdaloids are, as already 

 explained, porous rocks, into the cells of which mineral matter such as 

 silex, carbonate of lime, and other ingredients have been subsequently 

 introduced (see p. 469) ; sometimes, perhaps, by secretion during the 

 cooling and consolidation of lavas. 



In the Little Cumbray, one of the Western Islands, near Arran, the 

 amygdaloid sometimes contains elongated cavities filled with brown spar ; 

 and when the nodules have been washed out, the interior of the cavities 

 is glazed with the vitreous varnish so characteristic of the pores of 

 slaggy lavas. Even in some parts of this rock which are excluded from 

 air and water, the cells are empty, and seem to have always remained 

 in this state, and are therefore undistinguishable from some modern 

 lavas.f 



Dr. MacCulloch, after examining with great attention these and 

 the other igneous rocks of Scotland, observes, " that it is a me)e 

 dispute about terms, to refuse to the ancient eruptions of ti'ap the 

 name of submarine volcanoes ; 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 



* See Princ. of Geol., Index, "Graham Island," "jSTyoe," "Conglomerates, vol- 

 canic," &c. 



I MacCulloch, West. Islands, vol. il p. 487. 

 i S^st. of Geol. vol. ii. p. 114. 



