THE GEOLOGIST 



SEPTEMBEE, 1858. 



O:^ THE PEOBABLE Iiq^rLTJENCE TPOi^ THE INTERNAL 

 STRrCTURE OE ROCKS, OE THE MrTlFAL ERICTION OE 

 THEIR COMPONENT PARTS, WHEN EORCED INTO MOTION 

 UNDER EXTREME PRESSURES. 



By G. PoFLETT ScnopE, Esq., M.P., E.R.S., E.G.S., 8:c., &c. 

 (Bead he/ore the Geoloc/tcal Societijf May \2tk, 1858.) 

 In a paper read before the Geological Society on the 23r(l of April, 1856, 

 I referred to the proofs of a ''plasticity," or imperfect liquidity, in the 

 crystalline igneous rocks at the time of their protrusion, and called 

 attention to the mechanical changes, in texture and structure, which 

 could not fail to have resulted from the mutual friction of the 

 component crystals or granular particles of these rocks, during changes 

 of volume or of position, occasioned by variations in their temperature 

 while subjected to intense and irregular pressures. 



In illustration, I referred to the ''ribboned" pitchstoues and trachytic 

 lavas of Ponza, Ischia, Hungary, Mexico, &c., in which this structure 

 had unquestionably been produced in that manner, and I repeated the 

 opinion (to which more than thirty years back I had given expression), 

 that to such internal friction of the component crystals, was probably 

 owing the foliation of gneiss and mica-schist ; through the " squeeze 

 and jam" to which the lateral portions of an eruptive granitic axis must 

 have been subjected, between its own expansive force and the resistance 

 and pressure of the overlying strata. 



The Subject has so important a bearing on Geological Dynamics and 

 the doctrine of Mctaraorphism, that I trust to be excused for carryicg 

 the inquiry a little further. In doicg so it will be well to begin by 

 some elementary considerations. 



Jt is the well-known property of most substances to pass from a solid 



2 E 



