132 GEOLOGY. [chai\ vi. 



lie in continuous seams sufficiently distinct to justify the 

 idea that they have been separated and arranged by sub- 

 sidence. Meanwhile it is very probable that these materials 

 have not been projected into the air, but poured forth in 

 torrents of a consistence muddy or granular, in which dif- 

 ferences of specific gravity would exercise an imperfect ope- 

 ration. When we come to a mineralogical inspection of 

 specimens, we see further that the temperature required to 

 account for the formation of these beds must greatly have 

 exceeded that of the boiling mud thrown out by existing 

 volcanoes ; for this pumice differs from that which we find con- 

 joined with obsidian amongst subaerial volcanic products in one 

 remarkable respect — in containing, that is, prismatic crystals 

 of two kinds, the one colourless and transparent, being pro- 

 bably glassy felspar, the other black and opaque, believed to 

 be augite. The tufa, likewise, in which the pumice nodules 

 are embedded contains itself crystals of the latter kind, pre- 

 cisely the same as those which have been formed in the pumice. 

 The whole mass, then, at the time of its formation has been 

 subject to conditions in which very powerful disintegrating 

 and combining forces have been in simultaneous operation, 

 conditions foreign from that which has produced the vitreous 

 pumice of obsidian lavas. 



But another species of mineral occurs in these beds, both 

 at Madeira and Teneriffe, which throws light on the question 

 of the order and period of volcanic formation to which they 

 belong. In the tufas which have been cut through near Mr. 

 Veitch's quinta, at the Gorgulho, the author found, together 

 with sheets of carbonate of lime, plates and infiltrations of 

 Quartz resinite. The same opaline substance occurs similarly 

 in Teneriffe ; but the occurrence of opal, that is to say, of 

 a hydrate of silica containing water, as a large and essential 



