3 1 6 Mode of computing [Book VL 



Petrifactions are found upon, but not in, thcfe moun- 

 tains. 



Heterogeneous, or compound ftratified mountains, 

 confift of alternate ftrata of various "fpecies of ftones, 

 earths, fands, &c. The lime-ftone here is always of 

 the laminar, and not of the granular or fcaly kind, 

 and when it contains any ore, it is placed between its 

 laminae. Stones of the filiceous genus feldom form 

 flrata in thefe mountains, except lavas ; but the ftrata 

 are frequently interrupted by filiceous mailes, as jafper, 

 porphyry, &c. Coal, bitumen, petrifactions, and or^ 

 ganic hnprefiions, are found in thefe mountains, alfo 

 falts and calamine. 



There are other mountains, which cannot properly 

 be called ftratified, as they confift only of three im- 

 menfe mattes, the loweft granite, the middle argilla- 

 ceous, and the upper lime-ftone. Metallic ores arc 

 found in the argillaceous part, or between it and the 

 .lime-ftone, 



Confufed mountains confift of ftones heaped toge- 

 ther without order, their interftices filled with clay, 

 fand, and mica. They fcarcely ever contain any ore. 



Befides thefe, there are many mountains in different 

 parts of the world, which derive their origin from vol- 

 canoes;, but of thefe it will be neceffary to treat in the 

 fucceeding chapter. 



The height of mountains is ufually calculated by- 

 means of the barometer, upon the principles already 

 explained. For this purpofe two columns of mercury, 

 or barometers, are provided, and one is kept at the 

 foot of the mountain while the other is carried to its 

 fummit. The degree of heat, if not equal, is reduced 

 by calculation to an equality, and for this purpofe a 

 thermometer is attached to each of the barometers. 

 The degree of heat, to which both are reduced, is 



commonly 



