Geology. 231 



clay, from 60 to 80 feet thick, lying upon the white upper 

 limestone of the Great Oolite, and contains the celebrated 

 Pear Encrinite (Apiocrinus rotundus) which seems to have 

 once extended in a great colony over the bottom of the sea 

 at that time, as stems and roots of this crinoid are found 

 attached to the surface of the rock. It is very difficult to 

 obtain perfect specimens now of this interesting fossil, and 

 the magnificent collection which once adorned the cabinet 

 of Mr. Channing Pearce in the Circus, and was obtained by 

 him at so much cost and labour, has, it is to be regretted, 

 found a resting-place in London, far away from its local 

 habitat. The other fossils found in this clay occur also in 

 the overlying beds of the Forest Marble. 



This is comparatively thin around our hills, 

 rarely exceeding 100 feet, though it is much 

 thicker (450 feet) at Abbotsbury, in Dorset, and in the 

 Forest of Wychwood, whence it was named by Wm. Smith. 

 Some of the shelly beds, composed almost entirely of a small 

 ostrea, are compact enough to be polished, and used for 

 marble ; whilst the more flaggy beds have been utilized until 

 quite recently for roofing tiles. The numerous irregular pits 

 near Grenville's Monument, on Lansdown, indicate where 

 these fissile slabs have formerly been excavated. Lonsdale 

 has divided this formation into the following beds : — 



ft. 



I. Clay with occasional laminse of grit ... ... 15 



Sand und grit ... ... ... ... 40 



Clay with thin sla! s of stone ani laminae of grit ... 10 



Sh.'lly limestone or coarse Oolite ... ... 25 



Sand or sandy clay and grit ... ... ... 10 



Bradford clay 



The sandy beds called " Hinton Sands '' are largely de- 

 veloped at Charterhouse Hinton and contain concretionary 



