33 



pendent and peculiar polarizing effect on the rays, — a hypothesis 

 wliich appears necessary to account for the phenomena attending a 

 cliange in the disposition of the surfaces. 



9///, They possess a peculiar property, which the author conceives 

 •will render them available for the purposes of micrometry. When 

 the surfaces of the plates are parallel, two of the bands are united 

 into one at the centre ; but if a film be introduced between the plates, 

 so as to cause them slightly to diverge, the two bands in the centre 

 will be separated, and move laterally from each other, still preserving 

 their perfect parallelism. A film, -j^th of an inch in thickness, 

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

 every Tj^th of an inch of separation is equivalent to ^oooo ^^ °^ ^'^ 

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

 recourse must be had to the side bands, which are aflfected by a much 

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

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

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

 successive leaves of gold of a known thickness. 



2. Communication relative to the Fresh-Water Limestone 

 of Burdiehouse, near Edinburgh, belonging to the Carbo- 

 niferous Group of Rocks. By Dr Hibbert. 



lu his paper, the author explains that the limestone in question, 

 which is confounded with the common carboniferous or mountain 

 limestone of marine origin, is, in his opinion, of fresh-water origin. 



On an irregular line extending from Joppa on the coast of the 

 Firth of Forth, in a south and south-west direction to the Pentland 

 Hills, strata of mountain or carboniferous limestone crop out at 

 intervals; and their marine origin is indicated by encrinites, the 

 I'roductus, &c., and corallines. This limestone is developed with 

 least interruption between Edmonstone and Muirhouse, whore it is 

 from twelve to twenty feet thick. At this part of the line may be 

 seen fractures and elevations of the strata of limestone and superin- 

 cumbent shale and sandstone, evidently occasioned by a sudden and 

 violent uplifting force acting from north-east to south-west, and 

 causing the uplifted strata to dip south-east at an angle of 25°. 

 These uplifted beds, between Edmonstone and Muirhouse, and sub- 

 sequently to Burdiehouse, form the strata which dip under the coal- 

 measures of Gilmerton, Loanhead, and other sites. 



At Muirhouse Quarry the same mountain limestone is seen, but a 

 covered state of the ground succeeds in the same south-west direction 

 for a mile, in which no outcropping strata are observable, except some 

 beds of sandstone about the middle of that space, dipping with the 



