218 Messrs, Buckland's and Conybeare's Observations on the 



tersection of strata disposed in horizontal planes with strata disposed in inclined 

 ones. A stratum of the horizontal series appears to occur most uniformly 

 when its planes^ on being continued from the district in which it prevails^ are 

 not interfered with or intersected by the planes of the inclined strata; and 

 such a stratum^ on the contrary, appears to be most frequently wanting, where 

 steep and prominent ridges are formed by the inclined strata projecting above 

 the level to which the planes of the horizontal stratum would extend. Where- 

 ever such ridges ascend above the prevailing level of a hoi'izontal stratum, we 

 may expect the more recent horizontal strata, whose prevailing level is more 

 elevated, to form immediate contact with the inclined rocks. 



From the total want of conformity between the two series of rocks, we have 

 already deduced the consequence, that the lowest of the overlying deposits 

 may be placed in contact indifferently with any one of the inclined rocks. We 

 may now derive the still more general consequence from this want of confor- 

 mity, that any member of one series may be in contact with any member of the 

 other. 



Of the red marl and of the lias the prevailing levels are such, that they form 

 moderate acclivities* within and without the coal-basins, and are rarely found 

 investing the highest crests of the older chains ; but the oolite at its great 

 escarpment maintains so high a level, that it forms lofty eminences within the 

 coal-district itself; and would also probably have been found on the loftier of 

 the ancient ridges ; but its course to the westward is very limited, and it 

 scarcely begins to reach the eastern extremity of Mendips, when its beds are 

 discontinued and terminate altogether. 



The great escarpment of the oolite forms the natural boundary of the coal- 

 district on the east. It runs from the Cotswold hills by Lansdown, west of 

 Bath, presenting a lofty chain between 800 and 900 feet above the sea. To 

 the west of the principal escarpment it throws out many insulated masses, of 

 which Dundry hill, situated to the south of Bristol, is 790 feet above the sea. 



The formations of the red marl and lias compose the covering or mantle 

 which immediately surrounds the frontier of the Bristol coal-basin. On the 

 south they fill the interval between the Mendip and Quantoc hillsf ; on the 

 west they are found extensively on the shores of the estuary of the Severn ; 

 on the north they occupy the plain extending from Berkeley to Gloucester; 

 and on the east they form a continuous band, interposed between the foot of 



* Lias, however, often occurs in Somersetshire at 500 feet above the sea. 



+ For a description of the rocks of the second series in this quarter, we beg to refer to 

 Mr. Horner's paper " On the Geology of the South-west part of Somersetshire," printed in the 

 3d volume of the Geological Transactions. 



