4 



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



sncli warings and contortions as are often exemplified in that 

 rock. Whoever will examine the tortuous way in which the 

 plates of mica envelope and bend round nodules of half melted 

 quartz or crystals of garnet in mica-schist,, will be convinced, I 

 think, that the whole mass has been subjected to great internal 

 movement and consequent friction in the direction of the layers 

 of mica, while under intense pressure, and in a comparatively 

 softened state, the mica being lubricated, as it were, by a vehicle 



of liquid or gelatinous quartz. Whatever fissures or cracks 



were formed during tliis movement in the semi-solid rock, or 



subsequently, so long as the silicate remained unconsolidated, 

 would be necessarily filled by it, and ultimately appear in the 

 ^hape of the quartz-veins so frequent in this class of rocks. 



Under this supposition gneiss and mica-schist would bear the 

 same relation to granite as the ribboned trachytes and schistose 

 lavas (clinkstone) to ordinary crystallized or granular trachyte; 

 and the quartz rocks associated with granite, represent the quartz- 

 ose trachytes of Hungary, Ponza, and the Andes. 



These views, developed bv me in 1825, I cannot but think, 

 deserve the attention of geofogists engaged in investigating the 

 origin of the so-called ^'plutonic" and " metamorphic" rocks. It 

 seems to me more probable that some process of this kind may 

 have metamor23hosed granite into the laminated rocks of plu- 

 tonic origin^ gneiss, and mica-schist, than that these rocks should 

 have been formed by the mere fusion and reconsolidation or 

 crystallization in place of sedimentary strata already laminated^ 

 according to the usual '*metamorphic" doctrine. I can under- 

 stand the clay-slates and other fine-grained schists to have been 

 formed through the mechanical disintegration of mica-schist, but 

 not mica-scliisl by the baking or melting and cooling of the 

 €lay-slates in place, in the manner suggested by Sir C. Lyell. 



In the formation of the clay-slates, perhaps, the action of heat 

 was not concerned (except as engendering the pressure to which 

 they have evidently been subjected), but that of water or an 

 aqueous silicate only. Still in their case also internal movements 

 and mutual friction of the component particles under extreme 

 and irregular opposing pressures have, I am convinced, had a 

 primary influence in occasioning that parallel arrangement of 

 the scaly and flaky micaceous particles to which tlieir slaty 

 cleavage is due. This, at least, was the conviction forced upon 

 my mind by a close examination of the fissile clinkstone of the 

 Mont Dor and Mezen, which is used for roofing-slate, and is in 

 its lamination and cleavage undistinguishable from many clay- 

 slates. And that opinion I recorded at the time in my ' Consid- 

 erations on Yolcaiios.'* 



See pp. 103, 144, and 202. 



y 



