PART I. CHAPTER VIII. 



117 



Volcanic Eocks. 



are, as already explained, porous rocks, into the cells of which 

 mineral matter, such as silex, carbonate of lime, and other in- 

 gredients, have been subsequently introduced. (See p. 97.) 



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 cha- 

 racteristic 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 eruptions of • 

 trap the name of submarine volcanos ; 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.:]: 



Although the principal component minerals of subaerial lavas 

 are the same as those of intrusive trap, and both the columnar 

 and globular structure are common to both, there are, neverthe- 

 less, some volcanic rocks which never occur as lava, such as 

 greenstone, clinkstone, the more crystalline porphyries, and all 

 those traps in which quartz and mica frequently appear as constitu- 

 ent parts. In short the intrusive trap rocks, forming the intermedi- 

 ate step between lava and the plutonic rocks, depart in their char- 

 acters 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 next chapter, what is said of the plutonic formations. 



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

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



i Ibid. 



