Vol. 57.] UPPEE GEEENSAND AND CHLOEITIC MAEL OF WILTSHIEE. 103 



sand cohere together in lumps or masses. Pecfen orbicularis occurs 

 in scores, and Catopygus columbarius, Holaster Icevis^ Bhynchonella 

 i/rasiana, and Ammonites varians are also very abundant. A list of 

 the fossils obtained is tabulated on pp. 114-19. 



(3) The Cornstones are a bed of hard calcareous concretions, 

 closely packed together in a matrix of grey sand ; these are covered 

 with a dark-brown coating, and are locally called ' cornstones.' 

 In shape they are mostly oval, and they run from 6 to 9 inches 

 in length ; they are covered with small Serjpulce, and occasionally 

 one is pitted by the boring of a lithodomous mollusc. Internally 

 they are greyish-white, and consist of the finest quartz-sand, 

 without any visible glauconite, cemented by crystalline calcite into 

 a hard compact stone. Between the stones a few fossils occur in 

 the sand, such as Catojjygus columbarius^ Discoidea subucula, Tere- 

 hratula bijplicaia, etc. ; but they are not so abundant as in the layer 

 above. 



Bed 4 is a compact greenish-grey sand-rock or soft sand- 

 stone, with some calcareous matter. This bed encloses irregular 

 lumps of whitish sandstone, which are dispersed through its whole 

 thickness, and do not readily separate themselves from the sand. 

 Notwithstanding this close association of matrix and concretions, 

 the latter are not merely indurated portions of the former, for the)'' 

 consist entirely of fine white quartz-sand cemented by crystalline 

 calcite; whereas the material of the bed itself contains many large 

 grains of glauconite, and still larger scattered grains of quartz. 



It is not easy to understand how lumps of one kind of sand 

 could have become embedded in another kind of sand, unless they had 

 been derived from some previously existing bed ; but there is no 

 trace of any such bed either here or in other parts of the district, 

 and they do not present the aspect of derived stones.. Moreover, the 

 occurrence of such lumps of fine sandstone in coarser sand or sand- 

 stone is not limited to this district : it is a noticeable feature in 

 the topmost bed of the Upper Greensand in Western Dorset and the 

 adjacent parts of Somerset.^ There the fine white sand sometimes 

 separates itself in nodules, but more often appears simply in patches 

 which merge on all sides into the surrounding coarser sand, and 

 the whole mass is cemented by calcite into a hard rock which 

 breaks independently through both finer and coarser portions. 



There is no marked plane at the top of Bed 4. The sandy 

 matrix which embeds the ' cornstones ' passes down into the sand 

 below, which for a few inches is stained by iron, and in this portion 

 some of the same fossils occur as are found above. Fossils are not 

 common, and become still scarcer in the lower part, Rhynchonella 

 grasiana being the commonest. The base of Bed 4 is clearly 

 marked, because there is a decided change in the character of the 

 sediment forming the bed below. 



1 See Mem. GeoL Surv. ' Cretaceous Hocks of Britain ' toI. i (1900) ' Gault & 

 Upper Greensand of England ' pp. 173, 177, etc. 



