94 Essay on Geology. 



shall find the actual state of things exactly that which we should thus, 

 (i priori, have anticipated that it must be — a vast bed of shingle, composed 

 principally of fragments of carboniferous limestone, in blocks of various 

 sizes, from masses of two tons to lumps no bigger than a nutshell, and 

 commonly more or less water-worn, forms the first horizontal covering, 

 overspreading the general surface of the highly-inclined coal strata, the 

 broken edges of which appear to have been first worn smooth, and as it 

 were planed down, by the action of the violent currents to which we have 

 alluded. This shingle bed is, near the Mendips, about a hundred feet thick, 

 but grows thinner as we recede from them. Tiiis shingle has become con- 

 solidated by the earliest deposits of the poecilitic or variegated marls, into 

 an hard calcareous conglomerate j* as magnesia abounds in it, it is usually 

 (from the mineralogical name for magnesian limestone,) called thedolomitic 

 conglomerate. This rock often forms a very tolerable brecciated marble, 

 and as such, has been used in the pillars of Wells cathedral. Ft may be 

 well studied along the north brow of the Nettlebridge valley, at the north- 

 east end of the Mendips, where its horizontal strata will be seen forming a 

 thick cap, covering the inclined coal strata; as this shingle bed would 

 naturally rest indifferently on any older rock which came in its way, we 

 see similar beds on Durdham Down, resting on the mountain lime ; they 

 nmy be examined very advantageously in walking along the new road 

 winding up the Down, from the west foot of St. Vincent's rocks ; we shall 

 here be especially struck by the vast size of the blocks in the lowest beds, 

 but find them diminishing, till reduced to small grains in the upper beds. 

 All along the valley of the brook called the Trym, between the opposite 

 ranges of Durdham Down, where the carboniferous limestone dips east, and 

 Kingsvveston Down, where the same rock di|)s west, we find the dolomitic 

 conglomerate resting, at Sneyd Park, &c. on the subjacent old red sand- 

 stone, which must here have an arched, or anticlinal stratification. The 

 junction of the horizontal and inclined strata is very strikingly displayed 

 in a cliff on the south side of the Avon, opposite Sneyd Park. The 

 poecilitic, or variegated sandstone, or rather marl, (for only some of tiie 

 lower beds consist of sandstone,) forms the deposit immediately succeeding 

 to the shingle bed, or conglomerate just described ; in onr district it does 

 not exceed two or three hundred feet in thickness, but in the midland 

 counties must be sevt.n or eight times as much. It presents no organic 

 remains, but abounds in sulphate of lime, (gypsum,) and sulphate of stron- 

 tian, (celestine) ; both occur abundantly at Aust Cliff, near the Old 

 Passage, where the formation may be well studied. Celestine also occurs 



* In some places this shingle bed seeras not to have come in contact with the 

 planes of the poecilitic deposits, but with those of the superincumbent lias ; in these 

 instances, a singular conglomerate of carboniferous lime, cemented by Has, occurs. 

 This especially prevails about West Harrington, along the south slope of the Men- 

 dips, between Wells and Shepton Mallet. 



