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 

 occuri'ence 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 the}' 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 ftivurable 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 are destitute of evidences of the presence 

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

 thei'e are several categories in which such rocks may be classed. 



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

 of life have disappeared in consequence of the metamorphic changes 

 which those rocks have undergone. Experience shows that such cases 

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

 morphic strata may be recognized, either by means of their 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 that we find the formation manifesting itself in its original condi- 

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

 apply the tei-m "Azoic," since we should clearly recognize the fact that 

 their azoic chai-acter 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 tlie 

 larger portion of the Potsdam Sandstone of Lake Superior is entii-ely 

 destitute of traces of life; yet, as there ai'e 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 were incompatible with the existence of life. 



