ORDER IV. HEXAHEDRAL ROCK-SALT. 37 



Compound Varieties. Dentiform and some other 

 imitative shapes, rare. Commonly massive. Com- 

 position* granular or columnar, the latter in most 

 cases parallel, sometimes curved. Size of the com- 

 ponent individuals various. Faces of composition 

 rough. 



OBSERVATIONS. 



1. The species of hexahedral Rock-salt has formerly 

 been divided into different sub-species and kinds. The 

 principles, according to which this sub-division has been 

 effected, are the geognostic relations and the mechanical 

 composition of its varieties, and therefore not founded upon 

 Natural History. Thus the varieties occurring in beds, 

 have been called Rock-salt ; such as are formed at the bot- 

 tom of salt lakes, or on their shores, Sea-salt ; and the for. 

 mer again have been divided into foliated and fibrous Rock- 

 *alt, according to their granular or columnar mode of com- 

 position. 



2. The hexahedral Rock-salt consists of 



Muriate of Soda 983-25. 



Sulphate of Lime 6-50. 



Muriate of Magnesia 0-19. 



Muriate of Lime 0-06. 



Undissolved Matter 10-00. HENRY. 

 The composition of the Muriate of Soda is expressed by 

 Na M 3 = 53-29 of Soda, and 46-71 Muriatic acid. It is very 

 easily soluble in water, remains unaltered if exposed to the 



* A remarkable kind of composition on a large scale has 

 been long ago known to occur in the Rock-salt of Cheshire. 

 Concentric layers of this substance, of different colours and 

 purity, alternate with each other, and produce globular 

 masses of a diameter of several yards. Several of these 

 globular masses are again enveloped in concentric layers, 

 common to them all, in a manner exactly analogous to the 

 composition observable on a small scale in Pearlstone, a 

 variety of empyrodox Quartz. 



