398 STRATIGEAPHICAL GEOLOGY 



The blue clays contain Amaltheus margaritatus, Belemnites 

 elongatus, Plicatula spinosa, etc., and at their summit is a bed of 

 calcareous sandstone known as the Starfish Bed, from its having 

 yielded two species of Ophioderma. The Marlstone is here very 

 thin. 



When followed northward into Somerset the whole group 

 becomes thinner, its thickness near Glastonbury being little over 

 200 feet, but still having a bed of Marlstone rock at the summit, 

 from 1 to 2 feet thick. Round the Mendip Hills the Middle 

 Lias becomes very attenuated, and is represented sometimes by 

 a few feet of marly shale, and sometimes by conglomerate resting 

 directly on Carboniferous limestone ; in other places it is over- 

 lapped by the Inferior Oolite. 



Towards Bath the normal succession is again found, but with a 

 thickness of only 50 or 60 feet. Northwards through Gloucester- 

 shire the beds thicken considerably, till near Stinchcombe the 

 Marlstone is 15 feet, and the underlying sands and clays are about 

 150. The Marlstone here forms a conspicuous rock -bed which is 

 often quarried. It contains Paltopl. spinatum, Belemnites paxil- 

 losus, Rhynch. tetrahedra, Pecten cequivalvis, and other fossils. 



Upper Lias. In Somerset and Dorset this stage may be 

 described as consisting of three unequal portions (1) the Junction 

 Bed, (2) the Upper Lias clays, (3) the Upper Lias sands. The first 

 is a bed of pale-grey argillaceous limestone (l to 2 feet thick), 

 which is welded to the top of the Marlstone and contains in 

 successive layers the characteristic species of three zones, viz. 

 Harpoceras falciferum, Hildoceras bifrons, and Harpoceras striatulum. 

 The clays or shales are about 70 feet thick and contain but few 

 fossils, but species of Dumortieria have been found in them. They 

 pass up into fine yellow sands containing large burrstones or lumps 

 of calcareous sandstone. Of such sand there is nearly 200 feet, 

 but most of it yields no fossils ; between 40 and 50 feet from the 

 top a band contains Dumortieria radians and D. Moorei, and then 

 come 30 feet with species of the Harpoceras aalensis type, and 

 these are succeeded by about 5 feet of sand with opalinoid forms, so 

 that the passage from one stage to the other here takes place very 

 near the top of the sands. 4 



In. Somerset, near Yeovil, there is a similar succession, but the 

 base is formed by a band of shaly clay containing many small 

 Brachiopoda such as Leptcena Moorei, L. Bouchardi, and Zellania 

 liassica. This is overlain by a bed of limestone with Harp, 

 falciferum, and remains of fish, Crustacea, and insects. Above are 

 blue clays succeeded by sands with Ammonites of the radians 

 type (Dumortieria}, and in the higher part of these sands a 



