120 THE CANADIAN NATURALIST. [Dec. 



but the chemical history of this carbonic acid, considered with 

 reference to its origin, its fixation in the form of limestones, and 

 and its influence on chemical processes at the earth's surface, are 

 points for the most part peculiar to the author, and, in part, now 

 brought forward for the first time. 



ON THE GEOLOGICAL STRUCTURE OF THE SOUTHERN PART OF 

 MINNESOTA. 



BY PROF. JAMES HALL, OF ALBANY. 



The object of this paper is mainly to show a clear and depicted 

 geological structure of formations of different age, over a large 

 part of Minnesota, heretofore regarded as deeply covered by 

 drift deposits. 



In going west from the Mississippi River at St. Paul, we pass 

 over the older Silurian formations of Trenton limestone, Magnesian 

 limestone, and Potsdam sandstone, which extend as far as the 

 lower bend of the Minnesota, at Mankato. Beyond this, in 

 ascending the Minnesota River, for more than one hundred miles, 

 no pakeozoic formations are at present known. Approaching the 

 Minnesota, at New Ulm, over the high prairie from the East, we 

 find frequent exposures of a metamorphic rock, having on its 

 weathered surface a syenitic aspect, which is in reality a quartzite, 

 of gray, variegated or reddish color. On the Minnesota River, at 

 Redstone ferry, these quartzites are found to have a decided dip 

 to the eastward or south-eastward, and we have an exposure of 

 one hundred and fifty or two hundred feet of thickness. 



Triassic. — Abutting against the upturned edges of these 

 quartzites of Huronian age, there is a series of horizontal strata, 

 consisting of red marls, reddish and variegated, and red and gray 

 limestones, which are referred to the Triassic system. 



Cretaceous. — Lying upon the latter formation, and likewise 

 horizontally stratified, is a series of marls, clays, sandstones, 

 and beds of earthy coal, having altogether a thickness of perhaps 

 two or three hundred feet. The sandstones contain fragments 

 of plants or trees, and leaves of the willow, poplar, liriodendron, 

 and magnolia, all of which are referred to the age of the Creta- 

 ceous formations. 



Prairie Formation. — Covering all these, except in the 

 river banks, and at intervals in the prairie, is the deposit of drift 

 and lighter soil, constituting the Prairie formation. 



