-^jw j-yn-- ^—.--' - - ^ ^j^" 



526 



THE AZOIC SYSTEM AND ITS SUBDIVISIONS. 



with the present inquiry ; namely, that while there may be rocks which 

 are azoic because no life existed when they were formed, there are 

 others which from the very nature of their mode of formation and 

 occurrence could not show any indication of the presence of life, even 

 although it may have existed on the same planet, and in the immediate 

 vicinity of the rocks in question at the time they were deposited or 

 placed in their present position. Thus, no one would expect to find 

 signs of life in lava, or in the granitic masses originally constituting a 

 part of the earth's crust, and which were formed at a time when the 

 temperature and other conditions could not have been favorable to the 

 development of life, but which may have been raised since their for- 

 mation into perhaps close proximity to strata replete with the remains 



of organized beings. 



On looking over the subject of the existence of ''Azoic'* rocks 

 that is, of such formations as arc destitute of evidences of the presence 

 of life at the time of their formation or deposition — we find tliat 

 there are several categories in which such rocks may be classed. 



1st. We may have strata, once fossilifcrous, in which the evidences 

 of life have disappeared in consequence of the metamorphic changes 

 which those rocks have undergone. Experience shows that such cases 

 arc not uncommon ; but that, usually, the geological age of such meta- 

 morphic strata may be recognized, either by means of tlicir position 

 with reference to strata of known age, or by tracing the metamorphosed 

 formation to such a distance from the source of the alteration in ques- 

 tion tliat we find the formation manifesting itself in its original condi- 

 tion. To such metamorphic strata we should never have occasion to 

 api)ly the term ''Azoic," since we should clearly recognize the fact that 

 their azoic character was something which did not originally belong to 



them. 



2d. Rocks may be azoic, even if laid down when life was existing on- 



the globe, provided the local conditions were not favorable to its devel- 

 opment at the particular locality under consideration. Thus, much the 

 larger portion of the Potsdam Sandstone of Lake Superior is entirely 

 des^titute of traces of life i yet, as there are occasionally —even if very 

 rarely — localities where this rock is fossiliferous, we should not think 

 of separating the fossiliferous portions from the non-fossiliferous, or 

 of calling them by any special name, in consequence of the ^absence of 



traces of life in them. 



3d. Again, rocks must necessarily be azoic, when formed or originat- 

 ing^ under such conditions as wore incompatible with the existence of life. 



