104 PEOCEEBINGa OP IHE GEOLOGICAL SOCIETY. [Jau. 11, 



temiit 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 distui'bed 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 J^igan, 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 CuUen 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, &g. 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 Eirth. The arenaceous 

 beds, which we may suppose to have been dej)osited 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 hiU 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 lime- 

 stone which here forms the bed of the river. The top of the ridge 

 which divides the Dullan from the Piddich 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- 



