322 The Geology of Devizes. 



globules and spicules, but the amount varies greatly, and thin slices 

 of the Devizes Malm have rather a confused structure on account of 

 the large proportion of other mineral ingredients. 



The Malmstone is, therefore, an interesting rock, for we learn 

 that at the time it was being formed this district was the bed of a 

 sea which was inhabited by a large colony of sponges. Other 

 creatures do not seem to have found the area a suitable abode, for 

 larger fossils are scarce, except a small sea- worm, which built a 

 coiled calcareous shell (Vermicularia) . 



The Malmstone is well exposed by the entrance lodge to Broadleas 

 House, and along the scarped slope of Devizes Old Park. It passes 

 up into a soft Micaceous Sandstone, which also contains some siliceous 

 spicules and globules, but is mainly composed of the inorganic 

 ingredients, Quartz, Mica, and Glauconite. This Sandstone is ex- 

 posed in many road cuttings near Devizes, and has yielded a large 

 number of fossils. Of these at least three good collections exist, that 

 of Mr. W. Cunnington, now partly in the British Museum and partly 

 in the Jermyn Street Museum ; that of the Messrs. Sloper, which I 

 arranged and named last summer in the Museum of this Society ; 

 and thirdly, that of the late Mr. H. Cunnington, which has been 

 purchased for the Oxford Museum. These fossils are important, 

 because it is only near Devizes and Urchfont that this part of the 

 series contains so many organic remains. It would seem that the 

 conditions which were favourable to the growth of the Sponges did 

 not suit the Molluscs and Echinoderms, but that when the former 

 died out large numbers of bivalve Mollusca took possession of the 

 sea-floor, while Ammonites of several kinds swam through the water 

 above. 



The Sandstone passes up into buff and grey sands, at the top of 

 which there is a course of hard dark grey Calcareous Sandstone. 

 This is the lowest rock bed seen in the road cutting south of Devizes, 

 and, as it was formerly quarried at Potterne for building-stone, it 

 may receive the name of " Potterne Rock." This rock has yielded 

 the same assemblage of fossils as the Sandstone below. The thickness 

 of beds from the base of the Malmstone to this rock is about 70ft. 



4. Grey and Green Sands. Above the Potterne Rock are a set of 



