CHAPTER III. 



ROCK FORMATIONS. 



SEDIMENTARY. 



ARCHEAN. 



The Archean rocks as developed in this district belong apparently to 

 the very oldest of the crystalline sedimentary rocks, and on this ground may 

 be considered as corresponding with the eastern Laurentian. As yet no 

 systematic study of the Archean formations in the Rock)- Mountain region 

 has been made in accordance with which the different developments of 

 Archean rocks may be classitied as regards tlieir age and correspondence 

 with the different divisions made by eastern geologists. In the reports of 

 the Survey of the Fortieth Parallel recognition was taken of the fact that at 

 least two distinct developments of crystalline sedimentary rocks are found 

 in the Rocky Mountain region. 



Of these, the one, consisting essentially of granites, mica and horn- 

 blende gneisses, and amphibolites, being evidently the older, was considered 

 to correspond with the Laurentian series; certain accessory occurrences of 

 norite and beds of ilmenite and magnetic iron further allied it to this for- 

 mation. 



The second class, which was supposed to correspond to the Huronian, 

 was found in rather limited development at Red Creek, near the Uinta 

 Mountains, and along the Wasatch Range, and consisted of mica schists and 

 quartzites, the former passing into paragonite schists similar to tliose of the 

 St Gothard, with chloritic and hornblendic rocks, in general of a less 

 perfectly crystalline structure than the former. The Archean rocks of fhe 

 Black Hills, which consist of a great variety of slates, phyllites, quartzites, 



