140 



AZOIC AGE. 



3. The prevalence of iron is another characteristic (remarked by 

 J. D. Whitney). This is seen in the abundance of the minerals 

 (silicates) containing iron, as hornblende, hypersthene, chlorite, 

 garnet ; also the reddish color of much of the feldspar ; also the 

 beds of iron-ore, which exceed* in extent those of any other age. 



4. There are none of the simple silicates of alumina. 

 Arrangement of .the rocks. — Although the Azoic rocks are 



mostly crystalline, they follow one another in various alternations, 

 like the sedimentary beds of later date. In the sections which have 

 been given, there are alternations of granite, gneiss, schists, lime- 

 stone, etc. ; and the dip and strike may be studied in the same 

 manner as in the case of any tilted sandstones or shales. The 

 following sections represent other examples ; and in them there are 

 beds of iron-ore, one hundred feet and upwards in thickness, which 

 are banded with siliceous layers and chloritic schist, showing 

 thereby a distinctly stratified character. Where most flexed or 

 folded, there is still a distinction of layers ; and it is owing to this 

 fact that the rocks may be described as folded ; for folds can be 

 identified only where the rocks are in sheets. This grand fact is, 

 then, evident, — that the Azoic rocks are in layers, as much as the 

 rocks of any later age. 



The following section by Logan (real in its general truths, although partly 

 ideal) exhibits well this fact. It presents to view a stratum of (a) white 



Fig. 140. 



b a b a b a a 



granular or crystalline limestone, many times folded, and interstratified with 

 gneiss and quartz rock (b) ; and over the same region (Grenville and adjacent 

 country, Canada) the limestone has been traced in linear and curving bands 

 corresponding to a series of folds. 



The following sections contain iron-ore beds among the alternations. In 



Fig. 141. 



Fig. 142. 



Fig. 143. 



a a b x i i 



fig. 141 (from the Michigan region, Foster & Whitney) the iron-ore, in extensive 

 beds (i, i), occurs between chloritic schist (a, a) and diorite (6) ; and the iron- 



