GENERAL PRINCIPLES. 
113 
progress of the life and of the physical conditions and changes deter- 
mining the life of the globe. 
T1 ms the formations are all grouped into a definite number of systems , 
corresponding to different ages. 
The annexed chart shows the relative position and age of the different 
systems, and their general characterization according to the predomi- 
nance of different organic forms. 
IGNEOUS ROCKS. 
The Igneous or granitic rocks of course occupy the lowest position in 
the chart, being the oldest, and therefore underlying all the other younger 
and derivative formations. 
This series of rocks nowhere occupies very extensive areas of the 
earth’s surface. The prevalent species are granite, syenyte, porphyry, 
&c. 
LAURENTIAN SYSTEM. 
The lowest group of stratified or foliated rocks is called Laurentian , 
after the Canadian Survey, in which is described an enormous develop- 
ment of them along the St. Lawrence, reaching a thickness estimated at 
more than 15,000 feet. 
These rocks are found throughout the length of the Appalachians, in 
the Ozarks, and quite widely in the Rocky Mountains, and also in other 
parts of the world. This is not only the oldest of the series of stratified 
rocks, but is also one of the most extensive, both in vertical thickness 
and in horizontal area. Most of the metainorphic rocks of North Caro- 
lina probably belong to this horizon. The prevalent species are granite, 
gneiss, syenite and other liornblendic rocks, dioryte and crystalline lime- 
stone; and these contain much magnetic and specular iron ore, frequently 
in very large beds; and beds of graphite are also common. 
IIURONIAN SYSTEM. 
The second system in the series is called Hurotiian, from an extensive 
area of these rocks on the north shore of Lake Huron. They also occur 
in Northern Michigan ; and a narrow' belt along the entire Appalachian 
range is supposed to belong to this system. 
The most abundant and characteristic species are silicious and argilla- 
ceous slates and conglomerates, micaceous and hornblendic schists and 
