NATURAL HISTORY. 3. 



. Every stratum, whether horizontal or inclined, ha* 

 an equal thickness through its whole extent : that is to 

 say, every bed, of any matter whatsoever, taken se- 

 parately, has an equal thickness through its whole ex- 

 tent ; for example, when the bed of stone in a quarry 

 is three feet thick in one part, it will have the same 

 thickness throughout : if in one part it is found to be 

 six feet thick, it will be so throughout. In the quar- 

 ries about Paris, the bed of good stone is not thick, 

 and scarcely 18 or 20 feet thick ; in other quarries, as 

 those of Burgundy, the stone is much thicker ; it is the 

 same with marble ; the black and white marble have 

 a thicker bed ; the coloured are commonly thinner ; and 

 I know beds of very hard stone, which the farmers in 

 Burgundy make use of to cover their houses, that are 

 not above an inch thick. The thickness of different 

 beds, therefore, are different, but each bed preserves 

 the same thickness throughout its extent ; in general 

 it may be said, that the thickness of the horizontal 

 strata is so greatly varied, that it is found from one 

 line and less to 1, 10, 20, 30, or 100 feet thick; the an- 

 cient and modern quarries which are horizontally dug ; 

 the perpendicular, and other divisions of mountains, 

 prove that there are extensive strata in every direction. 

 The various strata of which the earth is composed, 

 are not disposed according to the order of their specific 

 weight ; for we often find strata of heavy matters plac- 

 ed on strata of lighter. To be assured of this, we have 

 only to examine the nature of the earth on which rocks 

 are placed, and we shall find that it is generally cla}', 

 which is specifically lighter than the matter of the rock. 

 In hills and other small elevations, we easily discover 

 the base on which rocks are placed ; but it is not s(* 

 with large mountains, not only the summit is rock. 



