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wheeler's survey. — JONES'S SURVET. 



515 



Furtbcnnore, in the Central and Soutliern Ilocky Mountains wc thul 

 tliat a large portion of the lower diviyions of the scries of stratified rocks 

 is wanting. Thus in the Front Uange of Colorado the erystalHiie masses 

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

 In such cases, and in these regions^ wc 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 hi the cabinet, botweon metamorphic erup- 

 tive and metamorphic sedimentary rocks. 



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WIIEELEIl'S SUEVEY. 



In the Reports of the Geographical and Geological Survey west of the 

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

 no evidence advanced to pi'ovc the occurrence in the supposed Azoic 

 areas examined by that Survey of distinct geological formations. 



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

 1870, embracing poi'iions of North Central New Mexico and South Cen- 

 tral Colorado, Professor Stevenson says : 



'' No ubyolule evidence exists to settle the age of these rocks. ■ Litliologically 

 they hear close resomI)lance to the Laurentiau series of the east, and at more 

 northern exposures within the Kooky Mountain region they liave been re- 

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

 erate granite iunuediaLcly underlying the Carboniferous at many localities may 

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

 ment, p. 72.) 



JONES'S SURVEY. 



In a Eeport upon the Eeconnoissancc 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 thc^ oldest of these 

 rocks in a group nearly e(puvaleut to the Laurentian system of the East, hut it 

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



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