52 HISTORY OF 



If from this external surface we descend deeper, 

 and view the earth cut perpendicularly down- 

 wards, either in the banks of great rivers, or steepy 

 sea-shores, or, going still deeper, if we observe it 

 in quarries or mines, we shall find its layers re- 

 gularly disposed in their proper order. We must 

 not expect, however, to find them of the same 

 kind or thickness in every place, as they differ in 

 different soils and situations. Sometimes marl is 

 seen to be over sand, and sometimes under it. The 

 most common disposition is, that under the first 

 earth is found gravel or sand, then clay or marl, 

 then chalk or coal, marbles, ores, sands, gravels ; 

 and thus an alternation of these substances, each 

 growing more dense as it sinks deeper. The clay, 

 for instance, found at the depth of a hundred feet, 

 is usually more heavy than that found not far from 

 the surface. In a well which was dug at Amster- 

 dam, to the depth of two hundred and thirty feet, 

 the following substances were found in succes- 

 sion :* seven feet of vegetable earth, nine of turf, 

 nine of soft clay, eight 'of sand, four of earth, ten 

 of clay, four of earth, ten of sand, two of clay, 

 four of white sand, one of soft earth, fourteen of 

 sand, eight of clay mixed with sand, four of sea- 

 sand mixed with shells, then a hundred and two 

 feet of soft clay, and then thirty-one feet of sand. 



In a well dug at Marly, to the depth of a hun- 

 dred feet, M. Buffon gives us a still more exact 

 enumeration of its layers of earth. Thirteen of a 

 reddish gravel, two of gravel mingled with a vit- 



* Varenius, as quoted by M. Buffon, p. 358. 



