114 LIFE. 



The fluccan is the half-decomposed rock adjoining a vein. 



A horse is a body of rock, like the wall-rock in kind, occurring in the course of a 

 vein. 



A comb is one of the layers in a banded vein, — so called especially when its surface 

 is more or less set with crystals. A cavity in a vein set around with crystals is called 

 a geode. 



Country, country-rock, wall-rock are terms applied to the rock in which a lode 

 occurs. 



A reef, in Australian gold mining, is a large auriferous quartz vein. 



Selvage is a thin band of earthy matter between a lnfle and its walls, or the sharp 

 line of demarcation between a lode and the wall-rock. 



A branch or leader is a small vein striking out from the main lode. 



Fahlbands, in Germany, Norway, etc., are metalliferous belts or zones; they some- 

 times consist of ore-bands (Erzbiinder), and rock-bands (Felsbander) ; or the lodes of 

 the region may be rich in ore only where they intersect the Fahlbands. 



On metallic veins, see further, Whitney's " Metallic Wealth of the United States" 

 (Philad. 1854), and Cotta's excellent " Treatise on Ore Deposits " (New York, 1869). 



The progress of the life of the globe is one of the two great sub- 

 jects that come before the student, in the following part of this Man- 

 ual, treating of Historical Geology. By way of introduction to 

 it, a short chapter on its system of structures is here introduced. 



BRIEF REVIEW OF THE SYSTEM OF LIFE. 



1. GENERAL CONSIDERATIONS. 



1. Life. — Some of the distinctions between a living organism and 

 inorganic or mineral substances have been mentioned. Recapitulating 

 them, with additions, they are : — 



(1.) The living being has, as the fundamental element of its structures, 

 visible cells, containing fluids or plastic material ; instead of invisible 

 molecules. 



(2.) It enlarges by means of imbibed nutriment, through a process of 

 evolution ; and not by mere accretion or. crystallization. 



(3.) It has the faculty of converting the nutriment received, into the 

 various chemical compounds essential to its constitution, and of con- 

 tinuing this process of assimilation as long as the functions of life 

 continue ; and it loses this chemical power when life ceases. 



(4.) It passes through successive stages in structure, and in chemistry, 

 from the simple germ to a more or less complex adult state, and finally 

 evolves other germs for the continuance of the species ; instead of 

 being equally perfect and equally simple in all its stages, and essen- 

 tially germless. 



There is, therefore, in the living organism, something besides mere 



