129 



2. Sands — not usually fossiliferous. 



1. Micaceous Sandy Shales and Clays with A. margantatus. 



These beds may be traced from the Dorsetshire coast inland 

 through the district north of Yeovil to Brent Knoll.* Whether 

 the junction of Middle and Lower Lias should be taken at the 

 base of the beds with A. margaritatus, or still lower, is a question 

 about which much diflference of opinion has been expressed ; but 

 it is usual now to group the beds with A. caprim-nus, A. 

 Eenleyi; A. Jamesoni, &c., wherever they occur, with the Lower 

 Lias. The Middle Lias is well developed on the Dorsetshire 

 coast, but it becomes much thinner towards the Mendip Hills. 



The determination of Middle Lias at the base of Brent Knoll 

 may be of some slight importance to those interested in the 

 prospects of Coal on tlie south of the Mendips ; at the Knoll 

 itself a considerable thickness of Middle Lias, as well as the 

 whole of the Lower Lias being present. The fault which 

 stretches from the neighbourhood of Wanstrow,t to the south 

 of Pilton, and brings the Lias north of Glastonbury against the 

 Eed Marls, is concealed beneath the Alluvium north of Meare, 

 and its continuance in the neighbourhood of Brent Knoll cannot 

 be indicated with any certainty. It is, however, evident that 

 Brent Knoll, like Glastonbury Tor, forms part of a basin-shaped 

 or synclinal structure in the rocks, and a greater thickness of 

 Lias might be expected in the immediate neighbourhood of these 

 hills than in the areas further away. 



Concerning the origin of the hill itself, the denudation is 



probably due in part to subaerial and in part to estuarine agencies. 



Knolls like Brent Knoll and Glastonbury Tor are met with in 



many other parts of the south-west of England, but nowhere 



* E. C. H. Day. Quart. Journ. Geol. Soc, vol. xix p. 278, and C. 

 Moore, Proc. Somerset Arch. Soc, vol. xiii., part 2, p. 119. 



t The inlier marked as "Great Oolite" on the Geological Survey 

 Map, at Wanstrow, is in reality Fuller's Earth. 



