306 GEOLOGICAL EXCURSION TO THE ROCKY MOUNTAINS. 



St. Paul and Minneapolis. The Silurian underlies the drift (occa- 

 sionally appearing through it) of a considerable area in the south- 

 eastern portion of the State just west of the Mississippi. 



The Devonian occupies a small portion of territory along the central 

 part of the southern border of Minnesota. It is chiefly represented by 

 the Corniferous (and Hamilton ?) limestones and is comparatively thin. 



The McHozoic — The only Mesozoie strata found are small isolated 

 areas of Cretaceous. These occur in basins in the older rocks and are 

 exposed in a few places along river courses. The Cretaceous rocks 

 contain many well-preserved fossil leaves, which have been described 

 by Leo Lesquereux. :! " 



The Cenozoir. — Nearly the whole of the State may be said to be 

 drift-covered; the only exceptions are the extreme southeastern and 

 the extreme northeastern portions. At any point on the northwestern 

 boundary, as far east as Lake of the Woods, one may start southward 

 and follow the Iowa boundary line without seeing any rocks in situ, 

 except what he might encounter in crossing the valley of the Minnesota 

 river, and the rare exposures of red quart/ate in Rock, Pipestone and 

 Cottonwood counties. East of this meridian he would encounter occa- 

 sional exposures of such rocks along Rainy river, but southward from 

 the northern boundary he would still have an almost equal scarcity of 

 rock exposures,were he to set out again to the Iowa boundary line. The 

 drift is so thick in the region of lakes IYniidgi and Winnibigoshish, and 

 generally through the central portion of the State, that it does not 

 afford rock exposures until reaching the vicinity of Motley. Rock 

 is seen in scattered patches in Todd, Morrison, Mille Lacs, Kanabac, 

 Stearns, Benton, and Sherburne counties, as well as at I'okcgama Palls 

 on the Upper Mississippi. But farther toward the south, except in the 

 valleys of the Minnesota and Bine Earth rivers, the drift everywhere 

 conceals the rock with an unbroken mantle from 100 to 200 feet, (30 to 

 00 m.) and sometimes 300 feet (01 m.) thick. 



East of the meridian passing through the west end of Rainy Lake, 

 the rock is more and more frequently seen projecting above the drift, 

 both along the Iowa boundary and in the central and northern por- 

 tions of the State, especially in the valleys of streams that flow east- 

 ward. There is a tract of the State heavily covered by drift east of 

 Pokegama Falls, including the St. Louis Valley audits upper tributa- 

 ries, in which many of the streams that enter Lake Superior in the 

 State of Minnesota take their rise; but for the most part in the eastern 

 half of the State the streams expose the rocks more and more fre- 

 quently, indicating an attenuation of the drift sheet toward the east, 

 so that at last they become continuously rock-bound. The drift Andes 

 out on the north toward the rock-bound shore of Lake Superior, as 

 remarkably evinced along the international boundary, and on the 



