ON CERTAIN CALCAREOUS NODULES. 

 By Professor E. G. Hogg, M.A. 



[Read 1st December, 1902] 



[Two photos.] 



The nodules described in the following paper occur in 

 certain bands in the upper members of the strata disclosed 

 at Duff's quarry, in Forster-street, New Town. The 

 nodule-bearing bands appear to be unfossiliferous, but 

 both above and below them are other bands carrying 

 numerous fossil impressions, including Alethopteris 

 australis, Thinnfeldia ohtusifolia, Phyllotheca, &c. The 

 whole series of bands is just below the horizon of the 

 New Town Coal Measures. The beds in which the nodules 

 are found are grey and blue-grey in colour, traversed by 

 thin lajT^ers of darker hue, thus giving rise to a banded 

 structure ; they may be described as a calcareous sandstone 

 of fine texture, fairly tough, and showing little or no 

 tendency to split along the planes of banding. 



In general the nodules have the shape of double-convex 

 lenses, giving in the most perfect forms a circle in plan and 

 two intersecting circular arcs of different curvatures in 

 elevation. The equatorial planes of the nodules were al- 

 most invariably parallel to the bedding planes; m the case 

 of those nodules having bounding surfaces of different 

 curvature the surface of greatest curvature was generally 

 found to be the one lying upp9rn:Dst. The larger nodules 

 have an equatorial diameter about four inches long, and 

 the lens at its thickest part is about half an inch through ; 

 on fracturing, the interior shows a crystalline structure, the 

 surfaces exposed having a steel-blue sub-metallic lustre. 



An analysis of the crystalline part of one of these nodules, 

 kindly made for me by M.x. Ward, A.R.S.IM., Government 

 Analyst, gave the following result : — 



Carbonate of Lime 44 • per cent. 



Silica, &c., insoluble in acid 50 • 6 „ 



Total Iron, taken as Peroxide... 4*7 „ 



Carbonate of Magnesia and loss. 0*7 „ 



100-0 

 The specific gravity is 2 5. 



