LIFE OF THE AZOIC AGE. 145 



Evidences of different epochs of revolution — that is, of uplift — may hereafter be 

 detected in the Azoic ; but thus far no definite progress has been made towards 

 this end, excepting in the separation of the Huronian proposed by Logan. 



Alterations: Solidification and crystallization. — Besides the 

 universal displacements, there was an almost universal crystalliza- 

 tion of the old sedimentary beds and limestones ; and now, in place 

 of the sands and clays and earthy limestone layers, the rocks are 

 crystallized into granite, gneiss, syenite, granular limestone, etc., 

 by the solidifying process, and thus have lost almost every trace of 

 their original sedimentary aspect. The once massive and earthy 

 limestones now contain in many places crystals of mica, scapolite, 

 apatite, spinel, etc., in place of their old impurities, and the lime- 

 stone itself is a white or variegated architectural marble. The 

 argillaceous iron-ore has become the bright specular and magnetic 

 ores, and it is banded by, or alternates with, schist and quartz, etc., 

 which were once accompanying clay and sand layers. 



Granite, syenite, and hyperite, although they present no lines of stratification 

 or foliation, are embraced among the metamorphic rocks as an actual part of 

 the series ; for the slight difference of structure between them and gneiss is no 

 evidence of difference of origin. Whatever crystals they contain besides their 

 own crystalline grains, — as of pyroxene, garnet, sphene, wollastonite, horn- 

 blende, feldspar,- -all were due to the same system of alteration. 



The upturning may have brought up also the granites, syenites, and other 

 rocks of a previous metamorphic period, or some of the nether granitic rocks 

 belonging to the first-formed crust ; but there is no means at present recognized 

 for distinguishing them ; and in many places the distinct alternation of the gra- 

 nite or syenite with the gneiss and schists proves beyond doubt their cotem- 

 poraneous origin, at the last great Azoic revolution, out of a common series of 

 sedimentary strata. That this revolution preceded the Silurian age is known 

 from the fact that the Silurian beds overlie them unconformably. 



It is remarked on a preceding page that some of the Azoic rocks 

 may have undergone a second or third alteration during the follow- 

 ing ages, and that the magnesian and aluminous potstones and 

 part of the serpentine and its associated minerals may be among 

 these later products. The mind that has any adequate appre- 

 hension of the remoteness of that Azoic era will not question the 

 probability of such changes, and is ready to wonder rather that 

 the evidences of subsequent alterations are not more extensive and 

 obvious. 



Life of the Azoic Age. — The term "azoic," as here used, implies 

 absence of life, but not necessarily of the lowest grades. 



The reasons in favor of the existence of life of some kind are — 



1. The formation of limestone strata in the Azoic age like those 



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