622 RELATION OF TRAP, LAVA, AND SCORIA. [Ch. XXIX. 



ous rock, cutting through, sheets of lava which may have spread over 

 the level bottom of the sea, and strata of tuff, formed of materials first 

 scattered far and wide by the winds and waves, and then deposited. 

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

 waves must give rise during the denudation of such volcanic islands, 

 will emerge from the deep whenever the bottom of the sea becomes 

 land. The proportion of volcanic matter which is originally submarine 

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

 tirely beneath the sea are almost all of them in islands, or, if on con- 

 tinents, near the shore. 



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

 appearances 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. 601) ; 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 ex- 

 cluded from air and water, the cells are empty, and seem to have 

 always remained in this state, and are therefore un distinguishable from 

 some modern lavas.* 



Dr. MacCulloch, after examining with great attention these and the 

 other 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 volcanoes; for they are such in every essential point, 

 although they no longer eject fire and smoke." f 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. J 



Although the principal component minerals of subaerial lavas are 

 the same as those of intrusive trap, and both the columnar and glob- 

 ular structure are common to both, there are, nevertheless, some vol- 

 canic rocks which never occur in currents of lava, such as greenstone, 

 the more crystalline porphyries, and those traps in which quartz and 

 mica appear as constituent parts. In short, the intrusive trap rocks, 

 forming the intermediate step between lava and the plutonic rocks, 

 depart in their characters from lava in proportion as they approximate 

 to granite. 



These views respecting the relations of the volcanic and trap rocks 

 will be better understood when the reader has studied, in the 33d 

 chapter, what is said of the plutonic formations. 



» 

 * MacCulloch, West. Islands, vol. ii. p. 487. 

 f Syst. of Geol., vol. ii. p. 114. 

 t Ibid. 



