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CHAPTER 111. 



ON THE MINERAL SUBSTANCES THAT COMPOSE THE CRUST OF THE 

 globe; AND ON THE STRUCTURE OF ROCKS. 



The constituent Elenaenls of the simple Minerals that compose Rocks. — The phys- 

 ical Characters of simple Minerals composing Rocks. — Explanation of the 

 Terms employed in describing the internal Structure of Rocks, and the external 

 Structure of Mountain Masses. — Sedimentary Depositions. 



The most careless observer can scarcely fail to notice, that the 

 mineral substances which occur on the surface of the globe differ 

 from each other in density, hardness, colour, and other sensible qual- 

 ities. Indeed, the different varieties of stone appear at first so nu- 

 merous, as to render it difficult to become acquainted with them : 

 but, however numerous these varieties may be thought, the simple 

 minerals which compose rocks or strata are very few, and the ele- 

 mentary substances, of which each of these minerals is formed, are 

 still fewer.* 



The elementary substances of which the solid matter of our globe 

 is composed, are the Earths^ — silex, alumine, lime and magnesia. 

 The Metals, — iron and manganese. The Inflammable Principles, 

 — carbon and sulphur ; and the Alkalies, — potash and soda. — Mu- 

 riatic and Phosphoric Acid occur also in the mineral kingdom. The 

 newly discovered earths and alkalies, and metallic ores cannot be re- 

 garded as forming essential constituent parts of rocks : they chiefly 

 occur in veins. The four earths above enumerated, together with 

 iron, compose nineteen parts in twenty of the known solid matter of 

 the globe. The Earths, when pure, are infusible, except at an in- 

 tense heat; they are nearly insoluble in water at the common tem- 

 perature : when pure, they are white or colourless. Though the 

 earths are infusible when pure, if they are combined in certain pro- 

 portions, they may be fused with facility at a comparatively low tem- 

 perature. 



* The mineralogist and the geologist consider those minerals as simple and 

 homogeneous, which present no difference of qualities to our senses throughout 

 the mass, although the chemist may discover that such minerals are composed of 

 two or more elementary substances. Thus, limestone or marble is regarded as a 

 simple substance, though chemistry has discovered that it contains, in every 100 

 parts, lime 57 parts, and carbonic acid 43. It is the latter which is expelled from 

 it by burning; a process which is well known to make the stone lighter, and to 

 render it caustic ; in which state it is called quicklime. Nor do the researches of 

 the chemist end here : the two substances, quicklime or pure lime, and carbonic 

 acid, are themselves compounds: the former, lime, is a compound of a metallic 

 substance called calcium, united with oxygen ; the latter, or carbonic acid, is com- 

 posed of oxygen, and carbon or charcoal. 



