76 Geological Society. 



duced, 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 press- 

 ure, — 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 accom- 

 panied by great internal movements and consequent mutual friction 

 among the component parts, and even among the individual cry- 

 stals ; 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 solid crystals, in the 

 manner before indicated. In this manner he supposes the foli- 

 ation or lamination of gneiss and mica-schist to have been produced 

 through the " squeeze and jam " of the lateral and superficial por- 

 tions 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 

 argues 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 the crystallific action will 

 have been freer in that than in any other direction. He likewise points 

 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 circum- 

 stances of imperfect liquidity, and irrespective of changes of tempera- 

 ture. And he suggests that to this cause may be attributable the 

 internal structure of some veined marbles, calcareous breccias, ser- 

 pentines, &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 concludes by suggesting to all geologists engaged in 

 the examination of rocks 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. 



