1871.] JA311ES0S BANFFSHIRE METAMORrHIC ROCKS. 103 



quartzose, almost a pure quartz-rock, with ferrugiuous stains along 

 the joints and planes of division. In Mac Culloeh's map it is by 

 some mistake coloured as part of the Old lied Sandstone, its 

 arenaceous character having probably iu some measixre led to 

 the error. As we proceed ujJ the course of the stream the rock 

 is seen to lie in gentle undulations dipping south-east at a low 

 angle, the quality still much the same, but rather more mica- 

 ceous — being a fine-grained micaceous quartz-rock, or quartzose 

 mica-slate. 'Nea.r 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 uO° or 35°. 

 The colour now becomes greener and the lamination piore 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 charac- 

 ter 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 hiU. The whole of this side 

 of the mountain from top to bottom consists of these rocks, indica- 

 ting a thickness of about 1200 or 1400 feet ; and as they seem to 

 extend across the Spey for some distance westward, the depth is pro- 

 bably 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 mad. 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 j-ear it is almost quite dry. 



Similar masses of rotten rock, approaching the consistency of soft 

 sandy mud, occur near the top of the Glenmai'kie 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 mould- 

 ering 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 Arudillv, on the west base of Ben Aigan. an at- 



