Page:Quarterly Journal of the Geological Society of London, vol. 27.djvu/217

 venie Castle, close by the edge of the stream. Here the slate troughs the quartz in a synclinal fold, which is much more abrupt at one side than it is at the other ; for as we walk along the river from north-west to south-east, we find the slate disappearing under the quartz at an angle of from 20° to 25°, and emerging again to the south-eastward almost vertically. The quartz-rock is here much crushed and disintegrated, as if by the nip it had got in the sharp curve of the synclinal fold.

The Granite — its Origin.

The granite of this region, I am inclined to think, has resulted from the fusion and recrystallization of the arenaceous beds. It is evident that the granite has originated after the deposition of these old sedimentary strata, because they are everywhere penetrated by its veins and injected masses, as may be well seen in the district around Lower Craigellachie. The granite, however, does not derange the strike of the beds to the degree that such a mass of foreign material should have done had it been erupted in an igneous condition, or forced up in any other conceivable way. I would rather suppose that the heat from the interior of the earth gradually approached the base of these sedimentary beds and, by heating, caused them to expand and thereby become wrinkled into huge folds, as a necessary consequence of a great mass of swollen matter having to find room in the space occupied by the same matter when in a cold and contracted state. The portions most liable to be fused would be softened and dissolved in situ, and be injected with enormous force, in consequence of the pressure, into all the openings and crevices around them. Crystallization would then take place as the whole very slowly cooled. In some such way, I imagine, the granite of this region has been formed out of the lower arenaceous and silty beds, and the greenstone of the Portsoy district out of the more argillaceous strata. The heat, as well as the watery vapour under such immense pressure, would probably penetrate further into the arenaceous beds than into the closer-grained clays. These views are confirmed by finding the granite occupying the room of what should have been gneiss or quartz-rock and the greenstone replacing the argillaceous beds. The serpentine of this region, as I have before mentioned, seems to have resulted from the metamorphism of beds containing much magnesia. In some places around Lower Craigellachie and the southern base of Ben Aigan, the gneiss is plentifully streaked with granite, as if partial fusion had just begun. These portions are found along the circumference of the great mass of granite, and seem to me to represent the gradual passage of arenaceous or silty strata by way of gneiss into granite. And here I may mention that the gneiss and quartz-rock of this region, even where most siliceous, always contains a proportion of felspar. The softening and fusion, as it progressed, would advance more rapidly along certain lines where the mineral matter was of such a nature as to yield most readily to the influence of the forces acting upon it.