THE PKODUCTS OF ACTIVE VOLCANOES. 
519 
terior 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. 
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 erup¬ 
tions of trap 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 im¬ 
probable that some of the volcanic rocks of the same country 
may have been poured out in the open air.f 
It will be seen in the following chapters that in the earth’s 
crust there are volcanic tulfs of all ages, containing marine 
shells, which bear witness to eruptions at many successive 
geological periods. These tuffs, and the associated trappean 
rocks, must not be compared to lava and scoriae which had 
cooled in the open air. Their counterparts must be sought 
in the products of modern submarine volcanic eruptions. If 
it be objected that we have no opportunity of studying these 
last, it may be answered, that subterranean movements have 
caused, almost everywhere in regions of active volcanoes, 
great changes in the relative level of'land and sea, in times 
comparatively modei’n, so as to expose to view the effects of 
volcanic operations at the bottom of the sea. 
* MacCulloch, West. Islands, vol. ii., p. 487. 
t Syst. of Geoh, vol. ii., p. 114. 
