Abstract

The author referred to a former paper read by him before the Society in April 1856, in which this subject was touched upon, and proposed to carry on the inquiry as to the probable effect, upon the internal structure of rocks, of the mutual friction of their component parts, when forced into motion under extreme and irregular pressures. He commenced by examining the laws that determine the internal motions of substances possessing a more or less imperfect liquidity, whether homogeneous, or consisting of solid particles suspended in, mixed with, or lubricated by any liquid, under unequal pressures; and showed that unequal rates of motion must result in the different parts of the substance, and that in the latter case there will be more or less separation of the solid and coarser from the finer and liquid particles into different zones or layers, those composed of the former moving less readily than those composed of the latter; and also that the former will, by the friction attending this process, be turned round so as to bring their major axes in the line of direction of the movements, and, if susceptible of tension or disintegration, will be elongated or drawn out in the same direction.

In illustraton of this law, specimens of marbled paper were produced, being impressions from superficial films of coloured matter floating upon water in circular or irregular forms after they had been subjected to motion in one or more directions by lateral pressure,—the appearances produced bearing a very exact resemblance to those presented by the lamination and occasionally sinuous or contorted structure of the ribboned lavas of Ponza, Ischia, the Ascension Isles, &c., as well as that of gneiss and mica-schist.

The author proceeded to state that the expansion of a subterranean mass of granite by increase of temperature, to which all geologists agree in ascribing the elevation of overlying rocks, must be accompanied by great internal movements and consequent mutual friction among the component parts, and even among the individual crystals; that, if a lubricating ingredient, such as water holding silex in solution, or gelatinous silex, be intimately mixed up with the more solid crystals (as there is great reason to believe to have been the case in granite), the friction will be lessened, especially in the central or inferior parts of the mass, where the expanding movement, or intumescence, may be supposed nearly uniform in all directions. But in the lateral and higher portions directly exposed to the resistance and pressure of the overlying rocks, shouldered off on either side by the expanding granitic axis, the movement will probably have been so predominant and extreme in a direction at right angles, or nearly so, to the pressure, as to give rise to a lamellar arrangement of the sold crystals, in the manner before indicated. In this manner he supposed the foliation or lamination of gneiss and mica-schist to have been produced through the “squeeze and jam” of the lateral and superficial portions of a granitic mass expanding by increase of temperature, and the giving way of the overlying rocks, those portions being forced to move in the direction of the lamination while subject to intense pressure at right angles, or nearly so, to that direction. The author argued that it is not inconsistent with this view, to suppose that a certain amount of recrystallization may have accompanied or followed this lamellar arrangement; in which case also the major axes of the crystals would be likely to take a direction perpendicular to the pressure, since the mobility necessary to tecrystallific action will have freer in that than in any other direction. He likewise pointed out that the influence of internal friction accompanying motion under extreme and irregular pressures must have been equally operative in the case of aqueous as of igneous rocks, under similar circumstance of imperfect liquidity, and irrespective of changes of temperature. And he suggested that to this cause may be attributable the internal structure of some veined marbels, calcareous breccias, serpentines, &c., as well as the cleavage of the slaty rocks,—as, indeed, the experiments of Mr. Sorby and of Professor Tyndall have already indicated. He concluded by suggesting to all geologists engaged in the examination of rock the above mechanical considerations, as likely to lead to more definite views than at present prevail as to the origin of the metamorphic schists, and the internal structure of many of the older and more disturbed rocks of all characters.