482 



PROCEEDINGS OP THE GEOLOGICAL SOCIETY. 



and abnormal conditions which deposits of the same age present 

 when they are more immediately in connexion with the Carboni- 

 ferous Limestone. When this is the case, they are found either rest- 

 ing immediately upon the limestone, filling up any basins or irregu- 

 larities in its surface (when there have been any opportunities), 

 passing down into its fissures, or lying against the southern outer 

 edge of what there is every reason to suppose was the ancient 

 Khfetic and Liassic coast-line presented by the Mendips at these and 

 subsequent periods. 



1. Marston Road Section. — Just above the hamlet of HolweU, in the 

 direction of Marston, a small roadside section of about 10 feet in 

 depth by about 30 yards in length, on the extreme south-eastern 

 exposure of the Carboniferous Limestone, affords the first indication 

 in this direction of the peculiar connexion of the Secondary with the 

 Palaeozoic rocks. At this spot large quarries of Carboniferous Lime- 

 stone in the valley below are to be observed ; and so precisely similar 

 in its lithological aspect is most of the stone in this section, that, 

 without an examination for its organic remains, it would readily be 

 supposed to be of Carboniferous-limestone age. In this very small 

 section, however, four geological formations are represented — namely, 

 the Carboniferous, the Hhaetic, the Lias, and the Inferior Oolite. 



In the lower portion of the quarry there is a capping of horizontal 

 Inferior Oolite 2 feet in thickness, rather dense and conglomeratic 

 at its base, but containing many organic remains of that age. How- 

 ever altered may be the condition of the beds below this formation, 

 the Oolitic rocks in this district, although they may be thinly repre- 

 sented and unconformable, are usually horizontal, and but little 

 altered in character. 



Immediately below the Oolite occurs a very dense unstra- 

 tified deposit of Liassic age, in part composed of the disjointed ossicles 

 of Pentacrinites, the matrix being a variegated yellow, pink, or 

 brown limestone. The presence of Gryphcea incurva, Belemnites, 

 Sjpirifera Walcottii, S. Munsterii (S. lata, Martin), TerehratnJa 

 punctata, T. Waterhousei (T. retusa, Martin), with UliynclioneUa 

 furcillata, U. variabilis and JR. snhvariahiUs, sufiiciently attest its 

 Liassic origin. It is, however, difficult to determine with certainty 

 the precise stratigraphical horizon to which to assign it ; but as 

 most of the above species are found in or above the Leptsena-bed of 

 the Camerton section, I am led to believe that the date of the deposit 

 is about that of the passage of the Lower into the Middle Lias, and 

 that it would therefore be on the horizon of the Pontaine-etoupe-Pour 

 beds, and of the Hierlatz-Schichten, which belief is strengthened 

 by the presence of Fecten verticiUus, P. Bollei, and P. paJosus, all 

 species from Hierlatz, whilst others, common to both, will be referred 

 to from Liassic veins in the quarries below. 



The Ehsetic deposits are represented in the western end of this 

 section by a, friable marl, about 10 inches thick, in which occur 

 teeth of Acrochis, Sargodon, &c., and vertebrae of Lepidotus, whilst 

 in the floor of the quarry the Carboniferous Limestone is seen. 

 To add to this remarkable variety, a mineral vein, 8 feet in thick- 



