Proceedings of the Roycd Society of Edinburgh. 1 9^ 



their perfect parallelism. A film, jjgth of an inch in thickness, 

 causes the central bands to separate to a distance of an inch, so that 

 every ^\yth of an inch of separation is equivalent to loi^nj^^ ^^ ^" 

 inch of thickness. When smaller thicknesses are to be measured, re- 

 course must be had to the side bands, which are affected by a much 

 slighter degree of divergence than the centre ones. A thicknesp so 

 minute as that of gold leaf may be rendered sensible by the side 

 bands, and a scale for micrometry might be found, by introducing 

 successive leaves of gold of a known thickness. 



1833, Dec. 16. — Sia Henry Jardine in the Chair. The 

 following communications were read : — 



1. *' A General View of the Phenomena displayed in the 

 Neighbourhood of Edinburgh by the Igneous Rocks in 

 their relations with the Secondary Strata ; with reference 

 to a more particular description of the section which has 

 been lately exposed to view on the south side of the Castle 

 Hill.'' By the Right Hon. Lord Greenock. 



The author, referring, in the introductory part of his paper, to the 

 views taken by Hutton of the structure of the earth's surface around 

 Edinburgh, explained, — That the prevailing rocks are strata of sand- 

 stone and shale of the coal formation, with occasional beds of lime- 

 stone ; and interrupted by insulated as well as grouped hills of igneous 

 origin, rising abruptly through them, — That the latter or trap-rocks, 

 seem in many quarters interstratified with the former, as if they 

 had burst while in a state of fusion between the strata of the second- 

 ary rocks, — That fragments of the secondary rocks are often seen 

 imbedded in the trap, as if they had been broken away from the 

 strata to which they belonged, and been hurried along by the fused 

 erupted mass, — And that the trap-rocks often present very different 

 appearances in the same hills, shewing that they were erupted under 

 varying circumstances at different periods of time. The author far- 

 ther explained, that the environs of Edinburgh seem to constitute a 

 great basin, surrounded by trap-rocks, which dip outwards in all di- 

 rections from a common centre, — the Pentland Hills forming the 

 southern boundary, the rocky coast of Fife at Burntisland the north- 

 ern, and Salisbury Craigs and Corstorphine Hill the eastern and 

 western limits. 



The paper then proceeds to describe the appearances presented by a 

 late section of the southern face of the Castle Hill, where several of the 

 phenomena referred to above are very well illustrated. The great mass 

 of the Castle Hill rock is a dark compact greenstone. Towards the 

 south-west point, altered rocks are seen resting on the trap in a highly 

 inclined position ; and within the Castle wall, fragments of sandstone 

 are imbedded in the greenstone, shewing that the latter must have 

 burst in a state of fusion through the strata of the former. But at 

 the south-east point of the rock, beyond the walls, the section lately 



VOL. XVII. NO. XXXIIT. — JULY 1834. N 



