Page:Quarterly Journal of the Geological Society of London, vol. 27.djvu/216

 tempt was made to work a small vein of ironstone which occurs there. On the top of the hill the rock is of purer quality ; and along the eastern slope it is covered by the clay-slate, which, however, seems to have its stratification more disturbed than at Mulben, and the junction is not so well exposed.

If the quartzose gneiss on the west side of the Spey comes out beneath the basement beds of Ben Aigan, as seems probable from the disposition of the strata, it would show that the thickness of the group of rocks lying beneath the slate is very considerable. The quartz-rock of Cullen lies on the line of strike of the Ben Aigan quartz, and is probably the northern extension of the same strata.

2. The Middle Division, or Slates. — With regard to the slate, the lower portion, as I have already mentioned, is well exposed to view along the line of railway at Mulben, and on the coast between Gamrie Head and Macduff there is a fine section. The limestone is exposed in a great many places along the river Dullan at Mortlach, the Loch of Drummuir, Auchendown Castle, &c. Its thickness varies a good deal in different places ; and in the coast-section between Gamrie and Macduff no limestone occurs, so that it would seem occasionally to thin out altogether.

The thickness of the slate lying between it and the upper quartz- rock I estimated as about 500 feet in some places ; and probably there is as much beneath it, which would make a total of 1000 feet for the mass of slates. But the thickness of the slate itself seems to vary a good deal, and to increase towards the coast. It probably consists of the finer sediment accumulated in deep still water, and would be thickest in the troughs of the old sea-bottom. We should therefore expect to find it thickening in certain directions. So far as I can judge, the thickness of the slate in this region seems to increase towards the trough of the Moray .Firth. The arenaceous beds, which we may suppose to have been deposited in water that was shallower or more traversed by currents, seem to thicken, or bear a greater proportion to the slates, as we go towards the interior of the country, as, for example, in the region of Braemar and Glentilt, where the quartz-rock is much developed and of great purity.

3. The Upper Division, or Upper Quartz-rock. — The meeting of the slate with the upper quartz-rock may be seen in some of the gullies that rut the side of the hill which forms the eastern bank of the river Dullan, in the neighbourhood of a place known as the Giant's Chair, a little way above the village of Mortlach. This Giant's Chair is an old pot-hole worn by the former action of the stream in the limestone which here forms the bed of the river. The top of the ridge which divides the Dullan from the Fiddich consists of the upper quartz, and so likewise does the top of that which separates the Fiddich from the Deveron. In the latter ridge, to the east of the old castle of Auchendown, the slate may be seen forming the base of the Glenmarkie Hill, and has been quarried for roofing purposes here and there along its western slope ; but the top of the ridge is of quartz. The slate may also be seen passing underneath the upper quartz-rock, on the eastern bank of the river Fiddich, opposite Bal-