POSITIVE ELEMENTS OF NORTH AMERICA 397 



brought out the great thickness of pre-Cambrian elastics derived from a 

 western land in British Columbia, and Dawson before him had described 

 the Shuswap and Adams Lake series as having similar relations* to an 

 Archean geanticline which occupied the site of the Gold ranges and ex- 

 tended as far north as latitude 57°. Although it is well laiown that the 

 metamorphic schists of the western part of the Cordillera are in consider- 

 able part of Paleozoic age and are not lightly to be reckoned as ancient 

 pre-Cambrian gneisses whose presence at the surface would indicate pro- 

 nounced elevation, nevertheless the sediments derived from a western 

 land and deposited in pre-Paleozoic or Paleozoic seas prove the existence 

 of a positive continental element in the Pacific region. Traced south- 

 ward through British Columbia into western Montana, the old land area 

 is lost under the post-Paleozoic intrusives and Tertiary volcanics of 

 Idaho and Oregon. South of the Snake Eiver flows it appears to have 

 occupied western Nevada and northern California, including possibly the 

 Klamath Mountain region. 



Proceeding northwestward beyond British Columbia, we find the Yukon 

 plateau, which according to Brooks is a region of deeply denuded early 

 Paleozoic or older rocks, flanked both north and south by folded Paleozoic 

 and Mesozoic strata. The central area may represent a positive element, 

 the backbone of Alaska, compressed between the thrusts from the Pacific 

 and Arctic basins. 



In Mexico and Central America there are areas of so-called Archean 

 rocks which might be interpreted as evidences of centers of elevation ; Init 

 the relations of unconformity are with rocks as late as the Mesozoic where 

 they are determined, and tliere is nothing to exclude the hypothesis that 

 the schists are metamorphosed Paleozoics, whereas the general descrip- 

 tions of the rocks by Ordonezf and Sapper| invite comparison with the 

 altered Paleozoics of the Great basin and California. It seems very 

 doubtful if there be a positive continental element in the peninsula region 

 south of Arizona. 



To sum up the enumeration of the positive continental elements, we 

 may name and characterize them as follows : 



Laurentia, the protaxis of Dana, the region of exposure of the most 

 ancient rocks, over which the sum of unconformities is apparently equal 

 to all post-Laurentian time and is to be reduced only by the epochs of 

 general submergence of Arctic lands. 



* G. M. Dawson : Geological record of the Rocky Mountain region in Canada. Bull. 

 Geol. Soc. Am., vol. 12, 1901, p. 84. 



t L'archaique du Canon de Tomellin, par Ezeq. Ordonez, Guide Geologique de Mex- 

 Ique, X Int. Congres, Mexico. 



t Uber Gebirgsbau und Boden des nordlichen Mittelamerika, C. Sapper, Peterm. Mitt. 

 Erganzungsband xxvii, no. 127. 



