§3. 



CRYSTALLIZATION. 195 



four-fifths of the atmosphere. Carbon is the chief con- 

 stituent of animal and vegetable structures, and is only found 

 pure in the diamond. 



There are also six metallic bases of alkalies and earths ; 

 namely, silicon, which, when united with an equivalent of 

 oxygen, forms quartz, or flint, the basis of nearly half the 

 rocks that compose the crust of the earth ; aluminum, 

 potassium, sodium, magnesium, and calcium; and two, the 

 oxides of which are neither earths nor alkalies, namely, 

 iron and manganese. The principal remaining metallic 

 substances, viz. copper, lead, zinc, arsenic, silver, gold, &c. 

 are comparatively unimportant in a geological point of 

 view. 



The sedimentary rocks are in a great measure composed 

 either of lime, silex, or argillaceous earth ; and accordingly 

 as these substances predominate, the masses possess, what 

 in mineralogical language is called a cleavage, or peculiar 

 fracture, which is distinct in each. Thus, if I take this 

 piece of flint and break it at random, it presents a glassy 

 or conchoidal fracture, and every fragment has a sharp 

 cutting edge ; and subdivide it as I may, each portion 

 retains the same character : but if I break this block of 

 chalk, the edge is not sharp, but blunt and dull, exhibiting 

 what is termed an earthy fracture. Again, if I shiver to 

 pieces by a blow of a hammer this calcareous spar, every 

 fragment partakes, more or less distinctly, of a rhomboidal 

 form ; so true is the remark, that we cannot break a stone 

 but in nature's joinings. 



3. Crystallization. — Crystallization may be denned a 

 methodical arrangement of the particles of matter according 

 to fixed laws.* For example — there are nearly 500 varieties 



* The definite or mathematical proportions in which substances 

 unite, both as to weight and volume, is a subject of the highest 

 interest, but which does not come within the scope of the present 

 inquiry. 



o2 



