282 Proceedings of the Royal Physical Society. 
When broken across, their lamine are found to be very hard 
internally, and bluish-grey in colour. 
The second bed (B), measuring about 2 feet vertically, 
consists of very hard pale bluish-grey flags, usually about 
14 inch in thickness, and 
yy aE yf also showing ochreous part- 
ge ings. Thaka lge, when nisi 
== taken out, are very hard and 
A == = difficult to split, but when 
4ft.2in. 2 exposed for some time on the 
SSS heap, they become rusty and 
SSS 
crumble away along the edges, 
while the weathering also 
tends to eat inwards, so as 
ultimately to split the whole 
up into thin rusty lamine, 
pi like those of the upper bed A. 
2 ft. 74 in. The difference between the 
two beds may therefore be only 
— one of degree of weathering. 
Fig. 2. Thirdly, the “ bottom” bed 
Diagrammatic section of the beds (C) consists of about 2} feet of 
exposed in Achanarras Quarry; rock, of a darker colour than 
acsle 2 Such to 2 foot. that which lies above, and 
which in the first place naturally divides into large broad 
flags, usually from 14 to 2 inches in thickness; but these, 
by application of hammer and chisel, readily split again 
into thinner layers, of which the thickness is only from 4 to 
? inch. 
From its dark colour this bottom rock is often called by 
the workmen the “black” rock, and, indeed, the partings are 
often so dark as nearly to merit that name, though, when 
broken across, the colour of the slabs internally is more of a 
bluish-grey of varying depth. In weathering, too, this rock 
behaves very differently from that of the 5uperincumbent 
beds, as it does not readily disintegrate on exposure, or 
become rusty or ochreous; on the contrary, it retains 
its hardness in spite of rain and sun, while the surface 
becomes of a light, often yellowish-grey, in fact a dirty 
B 
2 ft. 4 in. 
