G. P. Scrope on Craters, and the Liquidity of Lavas. 225 



character to a similar elongation of the feldspar crystals and feld- 

 spathic particles which ihey previouslj coutaine(3, in the direc- 

 tion in which the semi-liquid mass flowed, or rather was forced 

 to move, and in which the pores or cells, when there are any, 

 are equally elongated. These observations suggested to my 

 mind the reflection that the solid particles of any crystalline 

 rock which is put in motion while in a state of imperfect solidity, 

 and under the influence of opposing pressures, must be subject 

 to a great amount of mutual friction or disturbance, by which 

 their final arrangement when wholly consolidated will be deter- 

 mined. 



P Thus suppose a mass of granite, of which A B (fig. 4) repre- 



4. 

 C 



A 



D 



6. 



E G 



B 



sents the section, consisting of crystals of feldspar and mica irreg- 



U3 



silex, exposed to movement in the direction A B, while under 

 vast pressure both from above and below, that is in the opposite 

 directions C and D. Whether the surflice, C, or D, or both, re- 

 mained fixed, or merely moved, owing to resistances, at a slower 

 rate than the other parts, the crystals in the latter would be 

 turned round by internal friction, and rearranged and drawn out 

 in stripes or planes in the direction of the motion, while the pro- 

 portionate dimensions of the mass would be equally varied so 

 as to produce a section something like E, F, Gr, H,_ (%. 5), m 

 fact, a rock which, if no further change occurred m it except 

 consolidation in place, would have all the characteristics oi gneiss. 

 The same movement, if still further continued, might, it appeared 

 to me, be expected to disintegrate the angular crystals of felspar 

 altogether, so as to cause them to disappear, perhaps to force 

 their elementary molecules to melt into the intensely heated sili- 

 cate, to which they would impart their alkalies. And the result- 

 ing rock supposing- the lamiuc^ o? the mica-crystals to slide 

 i-eadily pLst each other, when lubricated by the silicate, and not 

 therefore to be so far disintegrated as those of feldspar (as from 

 their peculiar form might be expected), would put on a lamellar 

 structure and very much resemble Tnfca-sc/wsf,— especially since 

 the great flexibility of the mica would render its laminas ex- 

 tremely liable to yield to the irregularities of pressure pervading 

 the mass, in a variety of directions, and consequently to take 



SECOND SERIES, VOL. XXIV, NO. 71. — SEPT., J857. 



29 



