GENERAL PRINCIPLES. 
117 
rence of extensive and valuable seams of coal ; and while the strata of 
this series are generally poor in fossils elsewhere in the Atlantic slope, 
there have been found in this latitude many new and interesting organic 
forms, both vegetable and animal. 
JURASSIC SYSTEM. 
The rocks of this system, named from their extensive occurrence in 
the Jura Mountains of Switzerland, are not known in this country ex- 
cept on the Pacific Slope, in California and northward, and in some of 
the spurs of the Rocky Mountains. They are abundant in England and 
on the continent, and are notable on account of their remarkable fossils, 
the bones and teeth of enormous, walking, swimming and flying reptiles. 
CRETACEOUS SYSTEM. 
The name Cretaceous is due to the occurrence of the chalk beds of 
England in this period. These beds are a soft friable limestone, com- 
posed almost entirely of the shells of minute marine animals, and are 
formed, as the recent soundings and dredgings in the Atlantic and other 
seas have shown, onty in very deep waters. Chalk is scarcely found in 
this series in North America. The Greensand of New Jersey repre- 
sents the system, in part, on the Atlantic slope, and it is found also in a 
narrow zone, at a short distance from the coast, and at very ’moderate 
elevations above sea level, in the South Atlantic and Gulf States, from 
the Neuse River, round to the Mississippi ; but in a large part of this 
area it comes to the surface only in very limited tracts, being generally 
overlaid by newer formations. There is a vastly larger area on the wes- 
tern side of the continent which is occupied by rocks of this series, 
which there consists of sandstones, limestones, shales, &c., to the depth of 
several thousand feet, and contains extensive beds of coal. These rocks 
are also remarkable for the presence in them of the bones and teeth of 
enormous and uncouth reptiles corresponding in size to those of the pre- 
ceding (Jurassic) period in Europe. Many of the genera of living plants 
also occur first as fossils in the Cretaceous beds of the West. 
TERTIARY SYSTEM. 
This system, like the funner, is represented by a narrow zone of beds 
of no great thickness near the Atlantic coast, and spread out over a 
