G4 Introduction to Geology. 
merits of these systems, it appears most convenient, in the 
present instance, toadhere to terms long used and well under- 
stood ; and, in so doing, we hope to ayo 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. EHereafter, 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 pr inciples, 
they appear to have remained ex xactly 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- 
riner in a circuit of more than 260 leagues, and the eye takes 
