Intelligence and Miscellaneous Articles. 541 



Middle Carboniferous Group. 

 Stage E. Lower Coal-measures or Gannister Beds 1 jjasentiallv 



„ D. Millstone-Grit series V marine. 



„ C. Yoredale series J 



Lower Carboniferous Group. 

 Stage B. Carboniferous Limestone series ] Essentially ma- 



„ A. Lower shales, slates, Carboniferous and > rine (except 

 Calciferous Sandstone series J in Scotland). 



The author then proceeded to show, by reference to the writings 

 of Dr. P. Homer of Breslau, of M. De Koninck, M. Charles Bar- 

 rois, &c, that stage E with its marine fauna, is represented both 

 in Germany, Belgium, and Erance, as well as in the British islands, 

 so that the classification would hold good over Western Europe, 

 which was a sufficiently extensive area to justify the establishment 

 of a distinct group of strata. 



2. " On Coal-pebbles and their Derivation." By H. K. Jordan," 

 Esq., E.G.S. 



In this paper the author endeavoured to explain the mode of 

 production of pebbles of coal in the clays and sandstones of the 

 South- Wales Coal-field and elsewhere, the occurrence of which had 

 been long since noticed by Sir William Logan and Sir Henry De la 

 Beche. His opinion is that the pebbles in question are derived 

 either from the seam of coal above which they are found, or from a 

 seam of coal which formerly existed in the same, or approximately 

 in the same position, and which has been destroyed by erosion, the 

 effect of strong currents of water, which distributed the grains of 

 sand and other materials upon the coal-seam. 



LXXIII. Intelligence and Miscellaneous Articles. 



ON THE EMPLOYMENT OF A SILVERED GLASS AS A CAMERA 

 LTJCIDA. BY A. TERQUEM. 

 Tj^VEB,Y one knows how fatiguing is the prolonged use of the 

 -*- J cameras lucidse usually employed for drawing objects in relief or 

 microscopic objects. Nevertheless this apparatus is very convenient 

 when we wish to reproduce the outline of objects of which the per- 

 spective is difficult to obtain directly, such as certain physical appa- 

 ratus : photography cannot always be employed, when the sketch 

 is not to reproduce integrally the object itself with all its details. 



I have found that, for the usual camera lucida with either 

 one or two reflections, a glass semi-silvered by Eoucault and 

 Martin's process can be substituted with great advantage. Eor 

 this purpose it is sufficient to leave the glass in the silver bath from 

 one to two minutes at the most, according to the strength of the 

 reducing agent and, especially, according to the external tempera- 

 ture, the influence of which on the reduction of silver is consi- 

 derable. 



