416 NATURAL HISTORY. 



from the masses of half fused mineral matter spread 

 over them, and then suddenly cooled. 



They consist principally of Feldspar, Quartz, Mica, 

 and occasionally Hornblende, are frequently so crystal- 

 line in structure as to plainly exhibit the beautiful min- 

 eral materials which compose them. The metallic veins 

 found in this formation appear originally to have been 

 fissures, often passing through different beds of rock, 

 and which were subsequently filled with metallic ores. 

 They exist in primitive, transition, and secondary rocks, 

 but are most common in the former ; metallic veins often 

 change their metals at different depths and also their 

 dimensions.* 



The most ancient portion of this formation, the earliest 

 deposit on the first massive crust of the earth, is crys- 

 talline, namely, granite, which seems to form a basis for 

 the whole geological structure ; gniess syenite which 

 consists of feldspar, hornblende, and quartz : much re- 

 sembles granite; porphyry, serpentine, and greenstone, 

 the different varieties and properties of which shall be 

 described when we enter upon the field of special miner- 

 alogy. 



* Many theories are had respecting their origin. The earlier and 

 some of the modern geologists (Dr. Hutton, etc.) supposed that the 

 metals were forced into their veins in a fused state, the expansive 

 force of the heat producing their fissures. This is called the igneous 

 theory of the mineral veins. The aqueous theory, which is that of the 

 celebrated Werner and his followers, is, that the fissures of dykes 

 and veins were produced by the shrinking of the rocks in wliich they 

 are contained, and that the metallic veins were afterwards filled with 

 the metals in a state of solution poured in from the surface of the 

 earth. Tr. 



