Page:Quarterly Journal of the Geological Society of London, vol. 27.djvu/215

 quartzose, almost a pure quartz-rock, with ferruginous stains along the joints and planes of division. In Mac Culloch's map it is by some mistake coloured as part of the Old lied Sandstone, its arenaceous character having probably in some measure led to the error. As we proceed up the course of the stream the rock is seen to he in gentle undulations dipping south-east at a low angle, the quality still much the same, but rather more micaceous — being a fine-grained micaceous quartz-rock, or quartzose mica-slate. Near its junction with the slate the rock becomes more micaceous — a ferruginous-stained micaceous grit, alternating with seams of mica-slate, often thin-bedded and well laminated lying in regular order and dipping south-east, at an angle of 30° or 35°. The colour now becomes greener and the lamination more distinct ; and the passage upwards into the base of the overlying mass of slate is thus accomplished, there being thin seams of grit interbedded with the slate where it commences. The slate is here of a dull greenish colour, and is well exposed along the railway on to Mulben station, dipping south-east at from 30° to 40°. The mineral character of the group of rocks lying beneath the slate series may also be well studied along the western flank of Ben Aigan, where there are some deep gullies cutting far into the hill. The whole of this side of the mountain from top to bottom consists of these rocks, indicating a thickness of about 1200 or 1400 feet ; and as they seem to extend across the Spey for some distance westward, the depth is probably very great. In such a mass of sedimentary strata there must of course be a considerable variety in the quality ; and although the general character is quartzose, yet seams and beds of a softer and more slaty nature may here and there be met with. At one place I found the strata so rotten that considerable masses were reduced to the consistency of mud. The crushing, squeezing, and twisting to which the beds have been exposed have probably had something to do with this ; and the occurrence of such rotten beds here and there in a mountain must greatly facilitate the operation of those forces which carve out valleys and have removed such immense quantities of rock from the surface in many places. The rapidity with which the waste occasionally goes on may be seen in a deep gully or trench in the west flank of Ben Aigan, which seems to have been excavated by the action of a petty stream of water, so insignificant that at some seasons of the year it is almost quite dry.

Similar masses of rotten rock, approaching the consistency of soft sandy mud, occur near the top of the Glenmarkie ridge to the east of Auchendown Castle, in the upper quartz, although part of the strata in the immediate neighbourhood is a hard-grained white quartz- rock, or metamorphic grit. It is interesting to examine this mouldering bank of rusty brown sand and mud, containing some seams of disintegrated slate, where we see the rock reduced to something like what we may suppose to have been the original condition of the bed when it lay at the bottom of the ancient sea.

In some places the lower quartz-rock is much impregnated with oxide of iron ; and at Arndilly, on the west base of Ben Aigan, an at-