wheeler's survey. — JONES'S SURVEY. 515 



Furthermore, in the Central and Southern Rocky Mountains we find 

 that a hirge portion of the lower divisions of the series of stratified rocks 

 is wanting. Tims in the Front Range of Colorado the crystalline masses 

 are directly overlain by rocks generally admitted to be of Triassic age. 

 In such cases, and in these regions, we have the same difficulties which 

 occur in the Appalachian ranges; it is — or at least has been up to 

 the present time — very difficult for most geologists to clearly distin- 

 guish, either in the field or in the cabinet, between metamorphic erup- 

 tive and metamorphic sedimentary rocks. 



WHEELER'S SURVEY. 



In the Reports of the Geogi'aphical and Geological Survey west of the 

 one hundredth Meridian, under Capt. Geo. M. Wheeler, we have seen 

 no evidence advanced to prove the occurrence in the supposed Azoic 

 areas examined by that Survey of distinct geological formations. 



Of the rocks called "Archaean" in the region explored in 1878 and 

 1879, embracing portions of North Central New Mexico and South Cen- 

 tral Colorado, Professor Stevenson says : — 



" No absolute evidence exists to settle the age of these rocks. Lithologically 

 they bear close resemblance to the Laurentian series of the east, and at more 

 northern exposures within the Rocky Mountain region they have been re- 

 ferred by all observers to that age. The coarse gneissoid and often conglom- 

 erate granite immediately miderlying the Carboniferous at many localities may 

 possibly be of somewhat later origin." (Wheeler's Report, Vol. III., Supple- 

 ment, p. 72.) 



JONES'S SURVEY. 



In a Report upon the Eeconnoissance of Northwestern Wyoming by 

 Capt. Wm. A. Jones, in 1873, Prof. Theo. B. Comstock remarked on 

 the Azoic rocks examined by him : — 



" There are some slight reasons for placing a portion of the oldest of these 

 rocks in a group nearly equivalent to the Laurentian system of the East, but it 

 must be confessed that none. of them are based upon much better foimdation 



