64 Introduction to Geology* 



merits of these systems, it appears most convenient, in the 

 present instance, to adhere to terms long used and well under- 

 stood ; and, in so doing, we hope to avoid perpetuating that 

 confusion which the frequent changes of nomenclature and 

 classification in physical science necessarily produce. We 

 propose, therefore, in our brief sketch, as far as is consistent 

 with the present state of geological knowledge, to conform to 

 an arrangement hitherto generally adopted. Our enumeration 

 of the principal classes will naturally proceed according to the 

 order of creation or deposition. Hereafter, when we come to 

 detail the formations, we shall probably find it advantageous to 

 reverse that order ; and, commencing with the highest in geo- 

 logical position, descend to the lowest of the series. 



ORDER OF CREATION. 



Primary. 



Primary^ or Primitive^ Rocks, as their name imports, were 

 the first in the series of creation, the lowest in the order of 

 position. They are conceived to have originated prior to the 

 formation of any thing endued with life, because no traces of 

 any organic substances, animal or vegetable, have ever been 

 discovered in them. 



Their structure is crystalline, and, "in constituent principles, 

 they appear to have remained exactly in the same situation 

 as when they first acquired solidity." They occupy, probably, 

 not more than a sixth part of the absolute surface of our conti- 

 nents, but appear to form the foundation upon which all the 

 other rocks repose. They consist chiefly of granite, gneiss, and 

 mica slate, with horneblende, serpentine, sienite, &c., and 

 numerous modifications of each. They contain the metallic 

 ores, and primary limestone or statuary marble, and supply 

 the hardest materials for roads, and the most imperishable of 

 all stones for buildings, bridges, and other works requiring 

 great strength and durability. 



In England the primitive rocks are comparatively of insig- 

 nificant elevation, and are mostly covered with slate and other 

 rocks of later formation. 



In other parts of the world they attain to vast elevations, 

 and their height is further increased by volcanic products. 

 Gigantic masses, like the volcanoes of Teneriffe, of the Azores, 

 of Orozaba, and Caraccas, " insulated in the vast extent of 

 the seas, or placed on the coasts of continents, serve as mighty 

 sea-marks for the pilot, when he is unable to determine the 

 position of his vessel by observation of the stars." The Peak 

 of Teneriffe, for instance, " furnishes a direction to the ma- 

 i^iner in a circuit of more than 260 leagues, and the eye takes 



