696 W. J. MILLER PRE-CAMBRL\N FOLDING IN NORTH AMERICA 



The Engineer Mountain Quadrangle (Cross, 1910) contains Archean 

 gneiss and schist in its southeastern part with strikes varying from east- 

 west through northeast to nearly north-south, and Algonkian strata with 

 nearly east-west strike in its northeastern part. 



The Pikes Peak area (Cross, 1891:) contains some ]3re-Cambrian 

 gneisses with a north-northwest strike of foliation. 



Pre-Cambrian rocks in the Georgetown Quadrangle (Ball and Spurr, 

 1905) exhibit an excellent schistosity, with a prevailing east-west strike. 



Earlier pre-Cambrian gneisses in Eocky Mountain Park show a gen- 

 eral north-south strike of foliation, while steep-dipping Algonkian strata 

 in Thompson Canyon, just east of the park, strike about east-west. 



The southern portion of the Park Eange (Van Hise and Leith, 1909) 

 of northern Colorado consists of pre-Cambrian rocks with a prevailing 

 north-south strike, which changes to a general east- west strike in the 

 northern portion of the range. 



WYOMIXG 



Pre-Cambrian structural data are obtainable for several districts in 

 the southeastern one-fourth of this State. 



The Encampment district (Spencer, 1904), near the middle southern 

 border of the State, shows a large development of j)re-Cambrian strata 

 with a distinct strike nearly east-west for over 20 miles. This strike is 

 the same as in the adjacent Park Eange of Colorado. 



In the Medicine Bow Eange (Hague, 1811) a large development of 

 steep-dipping Archean gneiss, schist, and quartzite strikes about north 

 20 degrees east. 



In the Laramie-Sherman Quadrangle (Darton and Blackwelder, 1910) 

 of southeastern Wyoming pre-Cambrian gneiss and- schist are well devel- 

 oped, with a prevailing strike north TO degrees east. 



The Hartville district (W. S. Smith, 1903), in the eastern jjart of the 

 State, contains Algonkian schist, limestone, and quartzite witli practi- 

 cally vertical dips and a generally nearly east- west strike. 



A large area of pre-Cambrian granite is exposed in the Big Hor^ 

 Mountains (Darton, 1906). It is devoid of foliation, and so gives no 

 clew to the pre-Cambrian structure of this region. It is involved, with 

 Paleozoic strata on either side, in a gentle anticlinal structure. This 

 occurrence is of interest, however, because it is such a large body of pre- 

 Cambrian rock which has never been subjected to much pressure. 



Pre-Cambrian rocks are extensively exposed in the AVind Eiver Moun- 

 tains, but not much seems to be known about tlieir structure. In 1879, 

 however, Endlich said : 



