Page:Quarterly Journal of the Geological Society of London, vol. 27.djvu/214

 blackish hue. In the midst of this mass of clay-slate there is a bed of limestone (fig. 1 l) ; and both the limestone and the slate beside it often contain a large proportion of magnesia, and the varying metamorphism of these beds has in some localities given rise to masses of serpentine and talcose slate. This slaty series is covered by a second or upper group of arenaceous strata (fig. 1 c), often assuming the character of quartz- rock of a very pure white colour, forming the top of many of the hills of this district. Although in many places this upper quartz-rock is much worn away, yet in some of the mountains in the higher parts of the county it attains a great thickness, and forms the uppermost bed of the old metamorphic rocks exposed in this region.

We see therefore that the arrangement and succession of the metamorphic strata here are very similar to what we find in Bute and Argyleshire (see Quart. Journ. of Geol. Soc. 1860, vol. xvii. p. 133), namely a mass of clay-slate enclosed like a sandwich between two great masses of arenaceous beds. The rocks of Banffshire, in fact, seem to belong to the same formation as those of Bute. That is to say, they form the north-eastern extension of those beds which range through the central Highlands of Scotland, from the Moray Firth to the Firth of Clyde. In mineral quality and state of metamorphism the resemblance is very close.

1. The Lower Division, or Gneiss and Quartz-rock. — The passage from the lower quartz-rock upwards into the slates may be clearly seen along the course of a small stream, locally known as the Burn of Mulben, on the east side of the Spey, which is traversed by the line of railway from Keith to Elgin. Near the mouth of the stream, where it falls into the Spey, the mineral quality of the rock is, when freshly broken, white and

Fig. 1.-Section from the Spey to the Deveron, a distance of 12 miles.

a. The lower gneiss and quartz-rock.

b. The middle division, or slates.

c. The upper quartz-rock.

l. Limestone.