88 SCIENTIFIC AGRICULTURE. 



carbonic acid, 44, lime, 56, 100. It is a pulverizable rock, 

 of several varieties, which have resulted from the impurities 

 which were deposited with it. The chalk beds contain great 

 quantities of flint, which is dispersed through them in small 

 masses. Chalk also contains organic remains: it is a durable 

 building stone, and is used for docks, &c. ; some ancient 

 buildings are of chalk : no chalk has been found in America. 



ROCK SALT. 



This cannot be considered a rock, but yet it occurs in vast 

 beds, and in connection with rocks, at great depths in the 

 earth. In its pure form it is a transparent crystaline salt, 

 having the appearance of flint glass : the impure specimens are 

 reddish or bluish, and mixed with sulphate of soda and muriate 

 of magnesia. Its origin is not exactly known ; it is supposed, 

 however, to have resulted from the evaporation of sea water. 

 It is found in Spain, Poland, Hungaiy, Germany, and in some 

 parts of Asia and America. 



COAL 



Mineral or fossil coal is of several varieties, differing in 

 density and weight, and of a dark color, varying from brown to 

 jet black. It is composed of carbon and bitumen, and usually 

 contains some other matters. Coal is undoubtedly of vegetable 

 origin : as evidence of this, the organic structure of coal can be 

 seen in some specimens so distinctly that about three hundred 

 species of plants have been discovered in the various kinds. It 

 contains also many species of fossil animals. 



Coal beds vary in thickness, from a few feet to three thou- 

 sand or more, and are often several miles in length. The 

 manner in which such immense masses of vegetable matter 

 have accumulated during the lapse of ages, may be conceived 

 by reference to a single example. 



"According to Bringier, the quantity of timber which 

 drifted into the Atchafalaya, an arm of the Mississippi, during 



