In a paper read before the Geological Society on the 23rd 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” pitchstones 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 Metamorphism, that I trust to be excused for carrying the inquiry a little further. In doing so it will bo well to begin by some elementary considerations.

It is the well-known property of most substances to pass from a solid to a liquid state, and vico versâ, under varying circumstances of temperature and pressure.