12 The Geology of Cambridgeshire 



and yield an abundance of ammonites of the cor<lntn* 

 especially A. cordatus and A. Marlae, together with Belcm- 

 nites of various species and a great many lamellibranchs, The 

 stiff er and darker clays of the bottom of the pit con lain much 

 selenite. Two well-marked limestone bands are seen, but 

 should be avoided by the fossil hunter, being most unprofitable. 

 The best fossils are preserved in pyrites, and cavities within 

 them often contain beautiful crystals of barytes. In the 

 cutting between the pit and the station the two limestone 

 bands may be seen cut off by the Lower Greeusand uncon- 

 formity. 



At St Ives (Huntingdonshire), in Saint's brick pit, one mile 

 from the town, at the junction of the Chatteris and Ely roads, 

 beds quite similar to those at Sandy are well exposed, and here 

 also are very fossiliferous. The famous pit on the river side 

 north-east of the town is quite overgrown, and little but the 

 rubble fallen from the St Ives rock which topped the exposure 

 can now be seen. Two bands of sandy limestone were at one 

 time seen interbedded with the highest beds of Oxford Clay, 

 and that which appears near the top of Saint's pit is probably 

 the lower of these bands. 



At Holy well were formerly large brickyards which were 

 specially rich in fossils, and a well recently put down near the 

 ferry showed similar fossiliferous beds. In this well the 

 true character of the highest Oxford Clay was seen : it is a 

 very blue tenacious clay, which on weathering becomes light 

 grey-blue and ultimately passes into an almost creamy mud. 

 It is moderately rich in selenite and occasionally very 

 pyritous. 



The commonest fossils of this district are Ammonites 

 cordatus, A. Mariae, Belemnites hastatus, Waldheiniia /'///- 

 pressa, Rhynchonella nucula, and numerous lamellibranchs, as 

 Area, Nucula and Gryphaea. 



At Godmanchester is another pit which shows beds some- 

 what lower than those at St Ives ; fossils are there scarce. 



