GEOLOGICAL EXPLORATIONS AND LITERATURE— 1851. 27 



rucks l)C'l()\v tills line are evidences of intense and l()n<i--continued igneous 

 ag'ency, and in tliose above it proofs of comparative trantpullity and repose. 

 These pi-e-Potsdani rocks occupy an almost continuous belt along the north 

 slioi-e of Lake Superior, and are extensively developed in the southern shore, 

 forming the watershed between the respective river systems of Lake 

 Superior, Lake Michigan, and the Mississippi. The unconformable super- 

 position of the Potsdam sandstone of the Silurian system upun the (piartzites 

 of the Azoic sj'stem was seen near Carp River, where the last-named rocks 

 occur in ripple-marked beds standing nearl)^ vertical, while the sandstone 

 lies ai-ound it in nearly horizontal beds. 



The Azoic series was characterized by immense deposits of iron ore, 

 and the Silurian strata by deposits of coppei-. Near Teal Lake is a high 

 hill composed of alternating layers of jasper and iron ore that are curiously 

 contorted and plicated. 



It is impossible to form a correct notion of the thickness of the Azoic 

 series. If measured across the edges of the strata, we should have a 

 thickness greater than that of the whole fossiliferous series. The strata, 

 however, are plicated and folded, so that in measuring across their edges 

 the observer is passing over a repetition rather than a succession of beds. 



Foster, J. W., and Whitney, J. D. Report on tlie geology and topography 

 of the Lake Superior laud district. Part II. The irou regioD, together with the 

 general geology. Dated November 12, 1851. ;32d Congress, special session, 1851. 

 Senate Documents, Vol. Ill, No. 4. 400 pages. With plates and maps. Aljstract in 

 Bull. Soc. Geol. France, 1850, i)ages 89-100. 



A little later in the same year Foster and Whitney published the report, 

 which sums u]) all the information concerning the geology of the Lake 

 Superior region gathered by the authors during- their four years' connection 

 with the survey of the Chippewa land district, first as assistants of Dr. 

 Jackson, and during tlie last two years as the geologists in charge of the 

 survey. In this report the authors present an account of the geology of 

 the entire Upper Peninsula of ^Michigan. For the first time we here leam 

 of the general relations to one another of the various rock systems in this 

 region, and obtain the first definite information with respect to the detailed 



