6 CHARLES RICHARD VAN HISE 



pre-Cambrian series wherever there is a somewhat full succession of 

 formations is sufficiently great to make this an important factor in 

 the classification and correlation of the formation. 



lo. Degree of metamorphism. — -The amount of metamorphism 

 is a factor in correlation. Upon the whole, the older a series the 

 more likely it is to be metamorphosed, but this criterion has severe 

 limitations, since within comparatively short distances the closeness 

 of folding and the quantity of intrusives may greatly vary, and these 

 are very important factors in metamorphism. The worker among 

 the pre-Cambrian rocks must have a very thorough understanding 

 of the principles of metamorphism and the nature of the transforma- 

 tions through which rocks go. For, in working out the stratigraphy 

 of the pre-Cambrian, if the criterion of the original character is to be 

 used, it is necessary to know the rocks which the now greatly meta- 

 morphosed varieties represent, 



GENERAL STATEMENT 



In actually working out the succession of formations, series, and 

 groups in the different districts of a geological province and in corre- 

 lating them, all of the above criteria must be used. It is in judgment 

 in appreciating the value of each of these criteria and their combina- 

 tions that the skill of the pre-Cambrian stratigraphical geologist 

 appears. 



To this time, from my point of view, the only divisions of the pre- 

 Cambrian which have been proved to be general, if not world-wide, 

 are those of the Archean and the Algonkian. This subject I shall 

 not take up in detail, since I have recently discussed it in another 

 address.' 



However, it may be said in summary that the Archean is a group 

 dominantly composed of igneous rocks largely volcanic and for 

 extensive areas submarine. Sediments are subordinate. The Algon- 

 kian is a series of rocks which is mainly sedimentary. Volcanic 

 rocks are subordinate. The Algonkian sediments where not too 

 greatly metamorphosed are similar in all essential respects to those 

 which occur in the Paleozoic and later periods. When the Algonkian 



I Charles Richard Van Hise, "The Problems of the Pre-Cambrian," Bulletin, 

 Geological Society of America, Vol. XIX, pp. 1-2S. 



