ARCHEAN AND PALEOZOIC GEOGRAPHY 451 
has united with other elements, and entered into the rocks of 
later times, and since much oxygen has also entered into combi- 
nation in the earth’s crust, it seems probable that the air of the 
Cambrian time was charged with these two gases in very much 
greater percentage than we now find. It is possible also that the 
ocean was more impure, and that some of the beds of limestone 
which were formed in the Paleozoic seas, represent precipitations 
of these minerals from solution in the sea water. 
Ordovician Geography. — Both the Cambrian and 
Ordovician rocks of the east show such vast changes 
in many places, that little can be told concerning the 
conditions under which they were accumulated. There 
has been much metamorphism in these ancient strata, 
so that sandstones have been transformed to quartzites, 
limestones to marbles, and shales to slates. In fact, 
in some parts of New England, the alteration has 
passed so far that schists and gneisses have been 
formed out of these Paleozoic sediments. . 
The inland sea covered approximately the same 
area as that of the Cambrian; and until the close of 
the Ordovician, the geographic conditions of the coun- 
try appear to have been but a continuation of those 
of the preceding period. 
The close of the Ordovician was marked by a 
mighty uplift along the chain of eastern mountains. 
By the beginning of this period, the Archean chains 
had no doubt been worn in places to the condition 
of low hills, ike those now found in eastern Mary- 
