Yol. 6s.~] BASEMENT-BEDS OF THE BEISTOL COALFIELD. 451 



About 12J feet below the shell-bed occurs a third marine 

 horizon. This consists of about 3 feet 8 inches of black micaceous 

 shale, containing Lingula mytiloides in abundance and traces of 

 worm-burrows. There are thus four clearly-marked horizons, each 

 possessing a marine fauna. 



Recent examination of material upon the spoil-heaps at South 

 Liberty Pit, a colliery owned by the same company and situated 

 about a mile to the south of Ashton Vale, has shown the presence 

 there of goniatites, gasteropods, and Anthracomya Phillipsi in a 

 black shale. It has not been possible to localize these fossils as yet 

 in the underground workings ; but it appears likely that they have 

 come from a horizon not far removed from the ' Toad Yein,' and 

 therefore at an elevation of 500 to 600 feet above the highest 

 marine band in Ashton- Yale Colliery. It is, therefore, evident 

 that the unravelling of the fauna of the Bristol Coal-Measures has 

 not been terminated by the closing down of the Ashton-Vale pit, 

 but that higher horizons are still awaiting examination. 



III. The Millstone- Gkit Series. 



The general impression gained from the exploration-branch was 

 that it had passed, at some part of its course, from the Coal-Measures 

 into the underlying Millstone Grit ; and it therefore becomes neces- 

 sary to consider where the line should be drawn between them. 



The Millstone Grit bounds this part of the coalfield on the 

 north, and is exposed at Ashton Park, at Eownham Ferry, Brandon 

 Hill, also in road-excavations at Clifton, and was formerly exposed 

 in quarries in Tyndall's Park as well. At no place is the junction 

 with the overlying rocks seen. 



Anstie x stated that its thickness at Brandon Hill could easily be 

 determined, and estimated it at from 900 to 1000 feet. In the 

 Geological-Survey memoir 2 the average thickness for the Bristol area 

 is taken at 1000 feet, although it is pointed out that in the Mendip 

 Hills it cannot exceed 500 feet. The general conclusions as to the 

 thickness of the Millstone Grit near Bristol seem to have been in- 

 fluenced by the work of Fort-Major Thomas Austin, who in 1865 

 published a small work upon the subject. 3 In this work Major 

 Austin also described a long list of fossils which he had obtained 

 from the Grits and associated limestones, but there seems to be no 

 doubt that he placed the basement of the series too low, and included 

 some of the Upper Limestone Shales in his Millstone-Grit Series. 



A recent careful survey and levelling of a line of section from 

 the Summer-House Plantation in Ashton Park to Ashton- Yale 

 Colliery, taking in the line of the exploration -branch and based upon 



1 ' The Coalfields of Gloucestershire & Somersetshire & their Kesources ' 

 8vo. London, 1873, pp. 22-23. 



2 ' Geology of East Somerset & the Bristol Coalfields ' Mem. Geol. Surv. 

 1876, p. 27. 



3 ' The Millstone Grit, its Fossils, & the Eelation it bears to other Groups 

 of Bocks, &c.' 8vo. London & Bristol, 1865. 



