r 



222 G, P. Scroj)e on Craters, and the Liquidity of Lavas. 



tliese rocks at times exemplify. So also the transitions from the 

 larger crygtalHne grain to the finer, and the dykes and veins 

 which these rocks so often contain themselyes, or intrude into 

 their neighbors. So too the finer grains of the sides, orselv^ages, 

 of such dykes miglit be owing to the greater disintegration of 

 the crystals by friction along these sides as the matter was driven 

 through them. 



Another problematical fact which this theory of an aqueous 

 vehicle in heated granite would account for, is the usual appear- 

 ance of the quartz in this rock, not in crystals, but as a paste or 

 base, seeming to be moulded upon the crystals of feldspar. Had 

 the rock crystallized from a state of fusion, the feldspar, being far 

 more fusible than quart;^, might have been expected to bo the 

 last, not the first, to crystallize. But if the water disseminated 

 through the rock were supposed to have taken the quartz into 

 solution by aid of the alkalies present in the feldspar, the fluid 

 .vehicle would in fact become a liquid or gelatinous silicate; 

 and upon consolidation would naturally mould itself on the feld- 

 spar crystals, or appear as a paste to them, I adduced the hot 

 siliceous springs of Iceland and other volcanic districts as proofs 

 that heated water under such circumstances could dissolve silex. 



Those who will take the trouble to refer to the 2nd, 4th, 5th, 

 and 6th chapters of my ''Considerations on Volcanos," Avill see 

 that the above is a brief summary of the arguments there put 

 forth, perhaps at too great length, and in a form whicli may have 

 hindered their obtaining at the time of their publication the 

 attention which I believe they merited. 



Certain it is, that they \Yere at that time, now thirty years 

 back, neglected, or generally discredited. I was told that my 

 views were '' unchemicah" I was represented as asserting incan- 

 descent lava to be "cold or thereabouts,"* The igneous and the 

 aqueous origin of certain rocks had been so hotly contested, and 

 fire and ^vater were usually considered so antagonistic, - that it 

 seemed at first view an absurdity to imagine that both could be 

 combined in a substance seemingly in fusion. Probably also the 

 idea was scouted at first through the notion that w^ater could not 

 be present within an incandescent mass of lava without causing H 



it to explode hke a mine; which might of course be the result 

 of anv considerable body of water being localized at one point 

 But the view I entertained, as has been explained, was that the 

 water (and to some extent^ perhaps, liquefied gases), to which I 

 attributed much of the liquidity of some lavas, was disseminated 

 throughout its mass, occupying minute interstices, and in inti- 

 mate, though probably mechanical, combination with every 

 molecule,— mdeed intercalated between the plates even of its 

 solid crystals ; and moreover that the pressure to which the rocK 



* Westminister Review, 



