Geological Societi/. 531 



enced by that of his master Le Sage, a man of great originality and 

 profundity of thought, but whose speculations, particularly those 

 which attempted the explanation of the cause of gravity, trespassed 

 somewhat beyond the proper limits of philosophy. We consequently 

 find him disposed to explain the laws of the propagation of heat and 

 light on the most simple mechanical principles, and to trace their 

 origin and progress much farther than the experiments or facts will 

 properly warrant ; thus giving to his conclusions, in many cases, a 

 much more hypothetical character than would otherwise have at- 

 tached to them. M. Prevost had little acquaintance with the more 

 refined resources of modern analysis ; and his researches on many 

 important branches of experimental and philosophical inquiry were 

 consequently limited to reasonings which could be carried on by the 

 most simple algebraical, or geometrical processes. But notwith- 

 standing the restrictions which were thus imposed on his progress, 

 the range of his philosophical researches was unusually extensive 

 and various, and his discoveries on heat must always be considered 

 as constituting a most important epoch in a branch of science which 

 has recently received so extraordinary a developement in the hands 

 of Fourier, Forbes, Melloni, and other philosophers. 



GEOLOGICAL SOCIETY. 

 [Continued from p. 411.] 



May 22. — A paper was first read, " On the Wells found by dig- 

 ging and boring in the gravel and London clay in Essex, and on the 

 geological phenomena disclosed by them," by Dr. Mitchell, F.G.S. 



Essex consists chiefly of London clay, but that portion of the 

 county which lies to the north-west of a line drawn from Harlow 

 to Ballingdon Hill, near Sudbury, and the long ridge extending 

 from Purfleet to East Tilbury, are composed of chalk. Extensive 

 districts, however, are covered by thick deposits of gravel, sand, 

 and other detritus, varying in depth from 10 to 300 feet. In 

 Wakering Marshes and Foulness Island, there are 300 feet of 

 sand between the vegetable soil and the London clay. 



The wells formed in the gravel are supplied by land springs, the 

 water, when enough, being collected in a reservoir excavated in the 

 London clay. They are often not more than 12 feet in depth; 

 but it is impossible to estimate the number of feet to which they 

 must be sunk in any district, on account of the great inequalities 

 of the outline of the chalk. At Stanway, near Colchester, the clay 

 was found to be 45 feet from the surface ; but at the Union work- 

 house, less than a quarter of a mile distant, and on the same level, 

 it was necessary to sink 60 feet before it was reached. When the 

 London clay forms the surface there are no land-springs, as the 

 clay is generally impervious to water ; but in some places it is sandy, 

 and permits the percolation of water. That much of the rain which 

 falls in Essex penetrates downwards, is evident from the smallness 

 of the number and size of the brooks and rivers. Very little water 

 enters the Lea on the west side ; and into the Thames only four 



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