THE PROTEROZOIC ERA, 



209 



Though Archean rocks have not been identified in association with the Protero- 

 zoic system of this region, it does not follow that the latter were not derived from 

 the former, so far as they are sedimentary. The lands which gave rise to the 

 sediments appear to have sunk and been buried at some later time, and not again 

 exposed by erosion. The Proterozoic rocks of this region were long buried by 

 formations of lesser age. The uplift permitting the erosion which discovered the 

 older rocks is a relatively recent event (see Tertiary periods). 



Fig. 84. — Section through Kirk Hill in the Black Hills region, illustrating the type 

 of structure shown in Fig. 82. Legend same as in Fig. 82. Length of section, 

 3 miles. (Jaggar, U. S. Geol. Surv.) 



The Proterozoic system is represented in at least one locality in eastern Wyom- 

 ing. 1 The rocks are quart zites, schists, silicious limestone, and gneiss, and are 

 locally (near Guernsey) iron-bearing. They have also yielded some copper. 



In southeastern Missouri (St. Francis mountains) there are several small 

 areas of pre-Cambrian rock, both sedimentary and igneous. They are not asso- 



Fig. 85. — Section showing the relations o r the Tishomingo (I. T.) granite. Gr, 

 pre-Ordovician; 0: Ordovician (Viola limestone); S and 0, Silurian and Ordo- 

 vician (Sylvan shale); Cc, (Caney shale); Cw, (Wapanucka limestone); and 

 Ca, (Atoka formation^ Carboniferous; Kt, Lower Cretaceous (Trinitv formation), 

 Length of section, 5 miles. (Taff, Atoka, I. T. folio, U. S. Geol. Surv.) 



ciated at the surface with recognized Archean rock, and their relation to the 

 Proterozoic rocks of other regions has not bee a determined. As in the Black 

 Hills, the Proterozoic formations of this region were probably buried by younger 

 formations, and their exposure at the surface is the result of erosion following 

 subsequent uplift. The uplift in this area was probably much earlier than that 

 in the Black Hills. 



In Texas there is at least one series (Llano) of Proterozoic sedimentary rocks, 

 partly metamorphic, associated with igneous rocks which may be Archean. An 

 extensive area of granite (Tishomingo granite) in Indian Territory 2 is also believed 

 to be of pre-Cambrian age (Fig. 85). 



The Cordilleran region. — In this region the axial cores of many 

 of the older mountain ranges are believed to be of Archean rock. In 



1 W. S. T. Smith, HartviUe, Wyo., folio, U. S. Geol. Surv. 

 2 Taff, Tishomingo (I. T.) folio, U. S. Geol. Surv. 



