Page:Transactions of the Geological Society, 1st series, vol. 2.djvu/317

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and Sandstone at Stirling Castle Rock.

Transactions of the Geological Society, 1st series, vol. 2 plate page 0611.jpg

The sandstone bed itself consists of different laminæ, varying in colour, but generally slate-coloured, yellowish, and grey. The two lowermost and thickest beds differ little or nothing in texture, colour, or quality, from ordinary calcareous sandstones, and they are (as the drawing shews) of considerable thickness compared with those which immediately follow them.

These thinner beds are separated from each other by laminæ, of a clay slate, very much confounded with the sandstone, and those two substances alternate frequently and irregularly, till they approach the thicker beds which lie in contact with the greenstone. As they approximate to it they become more indurated, and assume a texture approaching to that of hornstone. The lowermost of the two upper beds thus exhibits a kind of hornstone of which the fracture is occasionally granular, and occasionally passes into the splintery and conchoidal. The uppermost bed exhibits the characters of perfect hornstone. It is of an ochre-brown colour, its fracture is conchoidal, or splintery, with an even shining surface, and the thin fragments transmit light.

As the specimens are before the Society, I need not enter into more minute details, particularly as, if taken together with the drawing, they will shew the regular gradation from the ordinary calcareous sandstone to the perfectly characterized hornstone. I may just be allowed to point out the resemblance which this change bears to that gradation from sandstone to a jaspideous rock, which occurs in a similar situation at Salisbury Craig. Considering merely the mechanical position of the sandstone, I confess myself unable to comprehend how by any combination of accidental abrasion or fracture, and successive deposition by precipitation, the present, and similar appearances can be produced. Although the projecting portion of the sandstone might have been able to maintain its place

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