614 THE AZOIC SYSTEM AND ITS SUBDIVISIONS. 



taiuing darker-colored micas. The intermediate system, to which the 

 name of Laiirentian is given, is said to be made iip of schists " composed 

 of quartz, feldspar, hornblende, and mica." This classification seems to 

 be purely a mineralogical one, and not justified at all — in so far, at 

 least, as the giving of the names Huroniarw and Laurentian is concerned 

 — by anything observed in the field. Indeed, Dr. Endlich himself states 

 that, 



" although special examinations were made to determine whether the different 

 mineralogical constitution of the [nietamorphic or Archaean] rocks remained 

 constant within certain zones, no applicable data upon this point could be 



obtained Within the main chain the stratoid segregation of the granites 



is not very completely carried out It would require the most careful 



examinations, conducted on a liberal scale, as to time, to elicit evidence bear- 

 ing upon the former condition of this metamorphic area." (Z. c, p. 66.) 



Much the same may be said, with truth, of the division of the Azoic 

 series into sub-systems by Dr. Hayden's assistants. Professor St. John 

 and Dr. A. C. Peale. The former assigns the crystalline rocks of the 

 Teton district to the Laurentian and Huronian systems, placing the 

 granites and gneisses in one division, and the quartzites and slates in 

 the other, there being no stratigraphical basis whatever indicated for 

 this arrangement. The reference to the Huronian is, indeed, suggested 

 only with doubt by Professor St. John. (/. c, p. 480.) The same is 

 true of the division of the Archaean into " Huronian'? " and "granite," 

 given by Dr. Peale. {I. c, p. 612.) 



It appears on examining the published volumes of the Hayden Sur- 

 vey, as well as from personal investigation of a part of Central Colo- 

 rado by one of the authors of the present paper, that through the whole 

 extent of the Cordilleras, and especially in the Rocky Mountain region 

 proper, the axes of the mountain chains are usually made up of crystal- . 

 line rocks, entirely destitute of fossils. These rocks are chiefly granites 

 and gneissoid granites ; slates occur in extremely subordinate quantity. 

 In the northern portion of the chain, where the entire Palaeozoic series, 

 including the Potsdam sandstone, is developed, there could be no hesita- 

 tion in assigning these when not eruptive to the Azoic or Archaean. But 

 it is not certain that there is any Archaean proper in the whole range of 

 the Cordilleras ; and in most cases it is beyond doubt true that the 

 axes of the chains have been erupted and the stratified masses uplifted 

 at a period much later than the Azoic. Thus, for instance, we have in 

 California post-Jurassic and post-Miocene eruptive granitic centres or 

 axes, in the Sierra iS^evada and Coast Ranges respectively. 



