Notice of Active and Extinct Volcanos. 277 



" The cellular aspect of the constituents of these submarine 

 lavas seems to shew that their age is, comparatively speaking, 

 modern, and with this the almost total absence of any Neptunian 

 products completely accords. 



" Sir G. Makenzie has noticed an effect of volcanic action of a 

 kind rather different from that which has hitherto come before us. 



" In many places, he says, an extensive stratum of volcanic 

 matter has been heaved up into large bubbles or blisters, vary- 

 ing from a few feet to forty or fifty in diameter. It also contains 

 numerous little craters, from which flames and scoriae had issued, 

 but no lava. These craters are often partially covered in by 

 domes of the same materials, as though the whole rock had been 

 first softened by the operation of heat; and portions of it had 

 then been made to swell outwards by the extrication of elastic 

 vapours. 



" Our author has chosen to distinguish this variety by the 

 name of cavernous lava ; its date is probably anterior to that of 

 the commencement of the present order of things, for it is in ma- 

 ny cases covered with gravel, and seems to extend under the sea. 



" When I come to speak of the Island of Lanzerote, I shall 

 have occasion to point out appearances described by Von Buch, 

 of a very analogous kind ; and shall therefore defer any attempt 

 to explain them for the present, proceeding in the mean time to 

 some other phenomena connected with the same subject, which 

 Iceland presents to our contemplation." 



The sulphur mountains of Krisiavik, consist of alternating 

 beds of white clay and sulphur, from all parts of which steam 

 is given out. 



" This was remarkably the case in a deep hollow into which 

 the author descended, where a confused noise was heard of boil- 

 ing and splashing, joined to the roaring of steam escaping from 

 narrow crevices. At the bottom of this hollow was a cauldron of 

 boiling mud about fifteen feet in diameter. There was a constant 

 sublimation of sulphur, which formed beautiful crystals round 

 the sides of the cavity. 



" The celebrated springs of Geyser are, however, the phe- 

 nomena which most forcibly arrest the attention of the traveller 

 in this country. The intermitting character of these fountains 

 may be, in some measure, imitated by pouring a stream of water 

 through a bent tube depressed about the centre, and heated in 

 that part alone. 



" Under these circumstances the steam suddenly generated at 

 bottom will force one portion of the water out in a jet from the 

 opposite extremity to that at which it entered, driving back at 



