466 THE AZOIC SYSTEM AND ITS SUBDIVISIONS. 



Potsdam sandstone in unconformable sequence. It was found by Pro- 

 fessor Cook that it was not practicable to divide the Azoic of ^'ew 

 Jersey, or to make of it " any other systematic classification than a geo- 

 graphical one." It will therefore not be necessary for us, in this con- 

 nection, to dwell upon the geology of the lower rocks in this State. 

 Since, however, this admirably conducted Survey is still in progress, and 

 it will yet be possible to obtain from it valuable additions to our present 

 stock of knowledge of that region, it will be well to call attention to 

 certain points in regard to which evidence is still lacking to prove the 

 correctness of the common belief about them. 



The relations of the different Azoic rocks to one another should be 

 most carefully studied, in order to find out whether the supposed gneiss 

 is, as a whole or in part, sedimentai-y or eruptive, the laminated charac- 

 ter not being svifficient to prove its origin, lamination being a very com- 

 mon character in eruptive rocks. The contacts of the rocks require 

 very careful examination, as it is only in this way that it will be possible 

 to make out the relative age, and in most cases the origin, of the rocks 

 in question. Of the rocks, the first one iu order of time should have 

 its origin determined by study of its intimate structure, and by compari- 

 son of it with similar rocks of known formation. It would be necessary 

 to remember that unconformability between the lamination or folia- 

 tion of an eruptive rock, and the stratification planes of a sedimentary' 

 one, means something very different from the unconformability of two 

 stratified deposits. Attention should be specially paid to the kind of 

 contact made by the supposed Azoic with the Palaeozoic, and examina- 

 tion should be made to see whether the latter at its base is composed 

 of debris derived from the supposed Azoic. The one case in which the 

 material of a conglomerate is referred to the gneissic rocks is a doubt- 

 ful one, and, if correct, would only show that the gneiss is older than the 

 Oneida conglomerate. (Geology of New Jersey, 1868, pp. 335, 336.) 



In the case of the iron ores associated with the Azoic, the evidence 

 given seems to be insufficient to prove the view either of their sedi- 

 mentar}' origin or of their eruptive character. Statements in regard to 

 the apparent conformable stratification and insensible passage of one 

 rock into another have been often made, and are easy to make ; but 

 such statements require proof based on most careful observation, made 

 by thorouglily skilled petrographers, before they can be accepted as con- 

 clusive evidence in regard to the question at issue. 



