306 Dr. FiTTON on the Strata below the Chalk. 



being applied to the same purposes in the construction of fire-places, &c. Very large pits of clunch 

 have been opened at Reach, on the immediate confines of the Fens, and on the line of Section 

 No. 23. 



Upper greensand. This formation here differs from that of some of the more southern counties, 

 in its much smaller thickness, in the absence of chert, and the comparative rarity of green 

 particles, which are here confined to a stratum not more than eighteen inches thick, by which the 

 lowest beds of the chalk are separated from the gault, as is well seen in the section exposed at the 

 Castle Hill, Cambridge. The presence of this green bed, however, is remarkably constant; 

 and it contains many fossils, some of which are common to this formation, and to the chalk, — 

 but others, at least in this country, are confined to the sand ; among which, Mr. Sedgwick 

 informs me, Hippurites are found near Bottisham. 



Gault. — The term Gault, or " Gait," adopted by Mr. Smith as the denomination of this re- 

 markable stratum, is in Cambridgeshire the popular name for the blue clay, which comes in 

 between the green bed last mentioned, and the ferruginous (Lower green) sand. Its thickness 

 and relations have been well ascertained in this county, from the numerous borings to obtain 

 water, which pierce through the clay to the sand below, especially on the line from Basingbourne 

 north-west of Royston, through Meldrith, and thence towards Cambridge. Its average thickness 

 is about 150 feet*, which is nearly the same as on the south-eastern coast: but the surface of 

 this stratum is much obscured by superficial gravel and a thin coating of the lowest chalk ; and 

 good specimens of its fossils are rare, as the upper beds contain but few. Among those, how- 

 ever, which I found between Cambridge and Ely, are the characteristic Ammonites of different 

 species, A . injlatus, A. lautus, A . varicosus ; Exogyra conica ; and spines of an Echinus. In 

 the museum of the Geological Society is a specimen from the Gault near Cambridge, which 

 M. Agassiz has found to belong to a new species of Chimaera -j-. 



Lower green-sand. — This stratum occurs throughout this county between the Gault and Kim- 

 meridge clay, in the form of sand of different shades of grey and brown, but chiefly as a coarse 

 ferruginous compound of quartzose sand, cemented by hydrate and oxide of iron, and more or 

 less indurated. At the top, however, is some green-sand, as appears from the first discharge from 

 the borings through the Gault, after the rod has passed the clay ; the water subsequently obtained 

 depositing an ochreous matter, of the colour of the Woburn sands. I have no fossils from this 

 stratum in Cambridgeshire. Its thickness is obviously much less than in Bedfordshire ; and its 

 internal composition was not discernible in any of the sections which I saw ; but at Ely I found 

 numerous concretional blocks, precisely resembling Kentish rag ; which had been obtained, I was 

 informed, from a place near the gallows on the south-west of the town, in a deposit which there 

 forms an extensive outlier over the Kimmeridge clay, and consists of coarse, ferruginous, and sili- 

 ceous sand and conglomerate, containing a large proportion of brown iron-ore (hydrous oxide of 

 iron), like that of Shanklin in the Isle of Wight. At Ingoldsthorpe in Norfolk, where the thick- 

 ness of the stratum is probably not greater than at Ely, it is said to contain Fuller's earth J. 



Quitting Potton, in Bedfordshire, Mr. Lunn states §, ferruginous sand is observed within the 



* Mr. Lunn supposes the thickness to be 200 to 220 feet. — Geol. Trans, vol. v. p. 115. 



f Proceedings of Geological Society, 1835, vol. ii. p. 205, note. 



X In iVIr. Greenough's geological map, sulphate of barytes is mentioned as occurring on the 

 main road from Royston to Huntingdon, atLongstow near Caxton. This mineral, it will be recol- 

 lected, occurs in the Fuller's earth at Nutfield in Surrey, ((51.) p. 141.); a situation which answers 

 to that of the Lower green-sand on the line of section No. 22. 



§ Geol. Trans. 1st Series, vol. v. p. 114, &c. 



