482 MINERALOGY. 



and Haliy, by directing particular attention to the geometrical 

 forms of the crystals, afforded bases for more precise specific 

 characters. These different modes of considering mineral bo- 

 dies have given rise to a great variety of opinion as to the pro- 

 per mode of classifying them. Berzelius and others contend for 

 an arrangement by which the species should be grouped in 

 conformity with their chemical composition ; others, as Wer- 

 ner, Hoffman, &c. reject the pure chemical and adopt a mixed 

 method, formed on the consideration of both external and che- 

 mical characters ; while Mohs and Jameson arrange mineral 

 bodies by their external characters alone. That system which 

 takes the whole structure and qualities of the objects into 

 view, is certainly the most philosophical ; but for the pur- 

 pose of ascertaining individual species, and placing them in their 

 particular Genus, Order, or Class, the external characters, as in 

 other branches of Natural History, are the most simple and 

 striking. The chemical combinations, requiring the assistance 

 of another science, though not less necessary to be known, fol- 

 lows of course the knowledge of the name and place of the body 

 in the series ; and this again leads to the consideration of the 

 use of the substance in the arts, and its situation in the struc- 

 ture of the earth. 



Mineralogy is divided into two great branches, viz. MINERA- 

 LOGY, properly so called, and GEOLOGY : the first treating of 

 the properties and relations of simple minerals ; the second 

 the various properties and relations of mountain rocks, or those 

 mineral masses of which the crust of the earth is composed, and 

 which are generally of a compound nature. In giving a slight 

 sketch of these two great divisions, the arrangement followed 

 by Professor Jameson in his Manual of Mineralogy * is chiefly 

 adopted. 



The external characters employed in the construction of the 

 principal divisions are, 1 . Form ; 2. Cleavage ; 3. Hardness ; 

 and 4. Specific gravity. 



1. FORM. The fundamental forms of minerals are four: 

 1. The Rhomboidal, or that in which the crystals resemble the 

 rhomboid in their general properties. 2. The Pyramidal,inv/}iich 

 the crystals assume the form of an isoceles four-sided pyramid. 



* Manual of Mineralogy : containing an account of Simple Minerals, and also a 

 Description and Arrangement of Mountain Rocks. Edinburgh, 1821. 



