ON VOLCANOES AND EARTHGLUAKES. 365 



volcanoes; and hence also, when mixed with water, 

 the eruptions of tenacious rnud. 



The rocks ejected in a liquid state, are the most im- 

 portant of the volcanic products, and are commonly 

 divided into obsidian, pumice, scoria or cavernous lava, 

 and solid lava; presenting great varieties of aspect 

 and composition. The distinctions between these 

 are not however always very definite; nor, in a geo- 

 logical view, are they of much importance. 



The consolidated currents of lava consist of a 

 greater or less variety of these rocks, and form irre- 

 gular beds, somewhat resembling strata when in par- 

 ticular positions. They are of various sizes, and often 

 very extensive. As they are frequently repeated in the 

 same spot, they are also found in irregular succession, 

 with layers of dust or puzzolana sometimes inter- 

 posed. In Iceland, they have occasionally been fused 

 in their places without flowing. Those which have 

 flowed under the sea, or under other more antient lavas, 

 are often compact ; while those which have run on the 

 surface, are porous or cavernous ; from that disengage- 

 ment of air which, in the others, has been prevented by 

 the superincumbent weight. In a few instances they 

 form columns resembling those of the trap rocks ; 

 and they also present examples of that laminar con- 

 cretionary structure so remarkable in trap, and which 

 occurs in granite also. 



The decomposition of lavas is very unequal in re- 

 spect to the time which it occupies ; and this arises 

 from the very variable nature of their composition ; 

 while they are noted, on ^Etna and Vesuvius, as well 

 as elsewhere, for producing, like the traps, remarkably 

 fertile soils. From false calculations respecting the 

 time required for this decomposition, and from mis- 

 taking interposed tufas for the effect of this, there 



