514} Geological Society : Anniversary Address. 



ally placed in single vertical rows, almost like the joints of a basaltic 

 column, sometimes nearly touching, but not articulating with one 

 another. 



A paper has been read by Mr. Henry Lawes Long on the occur- 

 rence of numerous subterraneous chasms or swallow-holes in the chalk 

 on the west of Farnham, with observations on the drainage of the 

 country near the west extremity of the highly-inclined ridge of chalk, 

 called the Hog's Back, between Guildford to Farnham. The land- 

 springs immediately on the north of Farnham descend southwards 

 in open gulleys over tertiary strata, until they arrive at the narrow 

 band of chalk which passes under Farnham Park, where they are 

 suddenly engulphed in transverse fissures or swallow-holes, through 

 which they pass under ground to a considerable distance, and again 

 break forth on the southern side of the chalk. Seven of these 

 swallow-holes occur near Farnham, from some of which the water 

 emerges in sufficient force to turn a mill. They are probably con- 

 nected with subterranean faults and transverse fractures, the origin 

 of which was coeval with the elevation of the narrow band of chalk, 

 which forms the Hog's Back, and which, near Farnham, is inclined 

 at a high angle to the north. The water that now passes through 

 the Farnham swallow-holes may tend to enlarge the chasms through 

 which it takes its subterraneous course, by dissolving slowly the 

 chalk of their sides in the small quantities of carbonic acid which 

 rain-water usually contains. 



Similar transverse fractures, on a greater scale, have given origin 

 to the chasms, which, being enlarged by denudation into transverse 

 valleys, afford outlets through the high escarpment of the chalk to 

 the rivers that, rising within the Weald, flow through the escarpment 

 of the north Downs into the valley of the Thames, and through the 

 escarpment of the south Downs into the sea, viz. to the Wey, the 

 Mole, the Darent, the Medway, and the Stour, through chasms in 

 the north Downs ; and to the Arun, the Aduz, the Ouse, and the 

 Cuckmere, through chasms in the south Downs. 



Dr. Mitchell has communicated a paper on Artesian and other 

 wells, in the gravel and London clay in Essex, showing that water 

 occurs under the London clay at various depths ; the deepest at 

 Foulness Island, being 460 feet. He attributes this inequality in 

 part to uneveness in the surface of the subjacent chalk. On 

 reaching the chalk a large volume of water usually rushes up. Ar- 

 tesian wells are now general in Essex, where they are of the greatest 

 utility in districts that have no natural springs. He also gives an in- 

 teresting list of localities, both of constant and intermitting springs, 

 some of them very powerful, that burst out from the chalk. 



Dr. Mitchell has also communicated an account of deleterious 

 gases that occur in wells in the chalk and strata above it near Lon- 

 don. The most abundant of these, namely, carbonic acid gas, issues 

 very partially and only from certain strata, and produces sometimes 

 effects fatal to persons employed in digging wells. Sulphuretted 

 hydrogen is occasionally met with in chalk ; and both sulphuretted 

 hydrogen and carburetted hydrogen occur in beds immediately above 

 the chalk. 



