THE FORMATIONS DESCRIBED. 333 



The lowest rocks considered in the following pages arc referred to those 

 now grouped as upper Cambrian. They will be discussed under two 

 divisions, as follows : 



•2. The Magnesian series = the Lower Magnesian limestones of Owen; 



1. The Potsdam sandstone = the Lower sandstones of Owen. 



The next higher group described is that commonly referred to the 

 Lower Silurian, including — 



5. The Cincinnati ; 



4. The Trenton ; 



:!. The Saint Peter. 



The highest rocks of the region described belong to the Devonian. 



Throughout the paper the rocks will be described in ascending order. 



Resume of earlier Investigations. 



This portion of Minnesota has been a favorite excursion ground for 

 the explorers of the Northwest since the time of Jonathan Carver. It 

 was he who first attempted any description of the rocks of this area. 

 He mentions that at the mouth of the Saint Peter river there exists a 

 bed of sandstone whose color is as white as the driven snow* Later 

 Lieutenant Pike,f Major Long,| and Featherstonhaugh § visited the falls 

 of Saint Anthony and many other places of geologic interest. The last- 

 named writer was the first commissioned geologist who ever visited 

 Minnesota, and while the compiler for Long's expedition made a section 

 of the strata at Fort Snelling. Lieutenant Allen || described the bluffs of 

 the lower Saint Croix from Stillwater to point Douglas, and J. N. Nicol- 

 let *[ published many desultory notes on this region which were, how- 

 ever, chiefly geographic. 



In the summer of 1839 David Dale Owen, of Indiana, received a com- 

 mission from the secretary of the United States Treasury "as the prin- 

 cipal auent to explore the mineral lands of the United States." His 

 instructions directed him "to proceed to Iowa and undertake an explora- 



*Travels through the interior parts of North America in the yours 1766, '67 and '68 : J. Carver, 

 Esq., hoi, lin, ivT'.i, ]>. 59. 



f Pike, .Major L. M. i An i ouni of expeditions to the sources of the Mississippi and through the 



western parts of Louisiana, etc. Performed by order of the government of the United States during 

 tin- years 1805, '6 ami '7. [Uustrated by maps ami charts. Philadelphia, 1810. 



I Narrative of an expedition to the source of Sain I Peters river, Lake Winnepeek, Lake of the 

 Woods.etc. Performed in the year 1823 * * '■■■ under the command of Stephen EI. Long, 1 . S- 

 T. E. William 11. Keating, J vols., London, 1825 (see p. 320). 



gA canoe voyage up the Minnay Sotor, by G. W. Featherstonl gh, -J \<>K, London, 1847 (see 



chaps, xwi-xxxvii. inclusive). 



American State Papers, vol. v, Military Affairs : Map and Journal of Lieut. .1. Allen, in charge of 

 escort i npanying Schoolcraft's expedition t.. tin- sources of the Mississippi, pp. 312-344. 



' Report intended to illustrate a map of tin' hydrographical basin "t the upper Mississippi river, 

 made by J. X. Nicollet February 16, 1841 (Senate Document 237, 26th ( ongn ss, 2d session). 



