THE GEOLOGIST 
SEPTEMEEE, 1858. 
O:^ THE PROBABLE INFLUENCE UPON THE INTERNAL 
STRUCTURE OF ROCKS, OF THE MUTUAL FRICTION OF 
THEIR COMPONENT PARTS, WHEN FORCED INTO MOTION 
UNDER EXTREME PRESSURES. 
By G. PouLETT ScKono, Esq., M.P., F.R.S., F.G.S., &e., &c. 
{Read lefure the Geological Societij, May \2th, 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 
crystalliuc 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" pitchstones and trachytic 
lavas of Ponza, Ischia, Hungary, Mexico, ka., 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 ; tlirough 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 ovcriying strata. 
The subject has so important a hearing on Geological Dynamics and 
the doctrine of Metamorphism, that I trust to be excused for carrying 
the inquiry a little further. In doing so it will be well to begin by 
some elementary considerations. 
Jt is the well-known property of most substances to pass fi-om a solid 
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