16 J. D. DANA — AREAS OF CONTINENTAL PROGRESS. 



thickness of the later sedimentary formations and the igneous outflows. 

 The Cascade chain, however, has an axis of granitoid and other crystalline 

 rocks for the most of tin- Sierra Nevada portion, which is probably Archaean 

 in time of origiu ; and the Archaean range is a long one, it' it extends, as is 

 reasonably urged by Mr. W. Lindgren, through the length of Lower Cali- 

 fornia. 



The intervening depressions, in the Pacific Border region, are first, the 

 Great Valley region, between the (.'oast and the Cascade chains, com- 

 prising the valleys of the Joaquim and Sacramento in California, the Will- 

 amette in Oregon, and valley-like depression between the so-called Coasl 

 Ranges of British America; secondly, the GREAT Basin region, whose 

 eastern boundary is the Archaean protaxis in British Columbia, but in the 

 United States, south of Montana, is a north and south line through the 

 Great Salt Lake, as shown by King; thirdly, owing to the bend of the 

 Archaean protaxis, widening so greatly the Pacific Border region, the United 

 States, south of Montana, has a Rocky-Summit region, which is the third in 

 the Beries of regions counting from the coast, while "Washington and British 

 Columbia have but two. 



The eastern limit of the Great Basin region, distinguished by King, divid- 

 ing it from the Rocky Summit region, is very nearly coincident with a south- 

 ward extension of the northern pari of the Archaean protaxis, or that north 

 <>f the bend ; and probably a series of Archaean ridges once continued along 

 this line, of which we have remains in the outcrops in and near Salt Lake, 

 including the high Archaean range along die Wahsatch Mountains, and in 

 other ridges farther south. Whether a continous range ever existed as a 

 western boundary or not, the" liocky Summit " region appears to be confined to 

 the United States, and to have well-defined limits— the western line extending 

 by Salt Like weal of south to the crossing of 1 1 1 ■ - parallel of 37 and the 

 meridian of 1 15 W., and then bending southeastward to the borders of Texas 

 and Mexico. West of the line for a long distance over the Great-Basin as 

 Kim:'- Report .-hows, the ( 'arboniferous rocks are the Latest ; directly easl of 

 it at many points begin the Cretaceous; and thus the distribution of the 



ii areas of Cretaceous on a map makes it generally easy to trace the 

 boundary, in spite of the great loss from erosion. 



Hut so far as the northern boundary is concerned the" well defined limits" 



made by the Archaean have, geologically, only a superficial value. The 



on i- actually continued, stratigraphically and orographically, into the 



bigh Rocky Mountain summit bell of British America, although thi- belt i- 



wholly east of the Archaean protaxis. 



The identity between the two regions, north and the other south of 



the Archaean bend, is apparent in several of their characteristics B >th are 

 on- of Paleozoic and Mesozoic rocks, in which the Cambrian, Carbonif- 



