218 SIR ROBERT CHRISTISON, BART., ON FOSSIL TREES 
grey hue, and a splintery somewhat glistening appearance, breaks with very 
sharp edges, is reduced to powder with much greater difficulty than the pure 
sandstone, and has a density of 2°63. Moreover, its surface, after being 
exposed for some weeks to the open air, acquires an ochrey hue, which in a 
few months becomes a rather lively ochre-yellow. These alterations led me to 
suspect their cause ; and analysis showed that the altered rock effervesces 
briskly in diluted nitric acid, loses weight in variable proportions, sometimes to 
the amount of 38 per cent., and yields to the acid a large quantity of lime, 
magnesia, and peroxide of iron. The iron, however, is dissolved as protoxide 
when hydrochloric acid is used, and atmospheric air excluded. In short, the 
sandstone must have been fossilised, so to speak, during consolidation by the 
same fluid which has fossilised the trees imbedded in it. When the quarrymen 
come upon this “ whin,” or “ bastard whin,” as they call it, they are apt to sus- 
pect the neighbourhood of a great fossil tree. But the same sort of altered 
rock is also met with, indeed too abundantly, in places far away from any fossil. 
It is bad stone for building purposes, as in time it becomes yellow, and in town 
black ; and it is the cause of the black patches which deface some of the build- 
ings of Edinburgh. It is easily distinguished, however, both by mineralogist 
and quarryman, so that the deformities referred to are inexcusable. The char- 
acters, whether external or chemical, are quite distinct when the proportion of 
of impurity is only 7 per cent. 
The fossils are imbedded without in general any material intervening between 
their black coaly crust and the sandstone matrix. There is in some places a 
narrow vacuity between them, probably produced by the concussion caused by 
the blastings for removal of the fossils ; possibly, too, arising from disturbances 
among the rocks at the time of their upheaval. At the concave part of the 
curve of No. 5 there was interposed, over a space of about a square foot, a 
narrow layer of slaty sandstone, containing black moulds of thin fern-like leaves 
and stems. A little farther up the same fossil, about a foot of the trunk had 
been crossed by a thicker band of rugged material, described to me as differing 
in appearance both from the coaly crust and from the matrix, and supposed by 
those who saw it to represent the remains of bark; but, unfortunately, it had 
been all removed before my visit to the quarry. 
The sandstone of Craigleith does not abound in other fossils. Black very 
thin moulds of leaves and stems of acrogenous plants are indeed common in 
slaty beds, which here and there are thinly interposed betwixt the thick beds 
of hard pure rock. WuIrTHAmM, too, mentions several small casts of sandstone 
fossil stems which had been preserved for him by the quarrymen. They. have 
been able to supply me with only two lepidendrons, one of them encrusted 
uniformly with caking coal, the other bare. Like all the specimens I have 
examined of fossilised plants from the coal-fields around Edinburgh, whose — 

