GEOGRAPHICAL DISTRIBUTION. 



135 



extent. These beds, thus crystallized and flexed or disturbed before 

 the Potsdam sands were deposited, are the Azoic. The following 

 sections illustrate this point. In each, the Azoic, numbered 1, in its 

 usual disturbed condition, is overlaid nearly horizontally by the 

 Silurian beds, 2 a being the Potsdam sandstone, 2 b the Calciferous 

 sandrock, 3 the Trenton limestones, 4 a the Utica shale. 



Fie. 137. 



Pig. 13S. 



Fig. 136, by Emmons, from Essex co., N.Y. ; 1 is bypersthene rock, or hyperite. — Fig. 137, 

 by Owen, from Black River, south of Lake Superior; 1 is a granitic rock, la, chloritic and 

 ferruginous slates. — Fig. 138, by Logan, from the south side of the St. Lawrence in Canada, 

 between Cascade Point and St. Louis Rapids ; 1, gneiss. 



Geographical distribution. — The Azoic rocks constitute the only 

 universal formation. They cover the whole globe, and were the 

 floor of the oceans and the rocks of all emerged land when animal 

 life was first created. But subsequent operations over the sphere 

 have buried the larger part of the ancient surface, and to a great 

 extent worn away and worked up anew its material; so that the 

 area of the old floor now exposed to vieAv is small. 



The areas of the earth's crust over which the Azoic rocks are now 

 exposed are either, — 



1. Those which have always remained uncovered. 



2. Those which have been covered by later strata, but from 

 which these superimposed beds have been simply washed away, 

 without much disturbance. 



3. Those once covered, like the last, but which, in the course 

 of the upturnings of mountain-making, have been thrust upward 

 among the displaced strata, and in this way have been brought out 

 to the light. 



In cases like those of figures 136, 137, in which the Silurian 

 rocks are spread in nearly horizontal layers over the borders of 

 an area made up of tilted Azoic rocks, the Azoic area either has 

 been always uncovered, or has become so from denudation ; but 

 in mountain-regions where the Silurian rocks have been folded 

 up in the mountain-making, the Azoic below may have been brought 



