RICE COUNTY. (555 



Trenton rocks.] 



The rocks of the Trenton period possess some characters that have been 

 ascribed to the Hudson River and Galena formations, where they appear 

 in southwestern Goodhue county, and these undoubtedly extend northwest- 

 wardly in Rice county, at least as far as to Cannon City, since the thickness 

 of rock, referable to the Trenton period, at the latter place amounts to 

 about a hundred and thirty feet. This is ascertained by aneroid measure- 

 ments from the top of the St. Peter sandstone in the Cannon river valley 

 west of Cannon City, combined with data learned from common wells at 

 Cannon City which encounter limestone at the depth of about thirty feet. 

 Nothing can be said of the lithology of these beds in Rice county, but the 

 elevated prairie under which they lie includes Richland, Cannon City and 

 Wheeling. These beds also probably extend with feathery edges into the 

 elevated tracts in eastern Bridgewater and southwestern Northfield. The 

 existence of a little lake at Cannon City is probably owing largely to the 

 impervious shales of this formation; and the long bogs which accompany 

 valleys of this part of the county are due to the same cause. 



The limestones of the Lower Trenton are well displayed in Rice county. They are abun- 

 dantly exposed along the valley of the Cannon river, and along Prairie creek, where they are 

 somewhat quarried. The thickness of these beds is about fifteen feet. They are overlain by a 

 heavy stratum of green shale, as in counties farther southeast, and there is a thickness of from 

 six to ten feet between them and the St. Peter sandstone. They embrace, along Prairie creek 

 valley, a carbonaceous layer of a few inches which, without previous drying, will ignite from a 

 common match and burn with a flame.* The Trenton also underlies the southern part of Warsaw, 

 extending probably into the southeastern part of Morristown. 



In general the Lower Trenton limestone is but little affected with magnesia or alumina as 

 impurities, in Rice county. It is compact, generally blue, and breaks sharply and somewhat 

 conchoidally. Its bedding is in sheets convenient for quarrying, being about six or eight inches 

 thick, and it is tolerably free from pyrites, though crystalline clusters of this are sometimes so 

 frequent as to cause a rusty stain on the surface of the blocks prepared for building. As quarried 

 at many places it is not blue, but has a faded ashen color, becoming also yellowish, but free from 

 pyrites, due to long weathering and submergence by the waters of the glacial period. 



Rice county affords the usual fossils that characterize this geological horizon, viz: large 

 orthoceratites, such as Endoccras magniventrum, II., several species of Strophomena, Orthis, and 

 of Rhynchonella, as well as specimens of a large coiled cephalopod like Lituites undatux, Con. t 



At Faribault the strike of the Lower Trenton, on the west side of the Straight river, passes 

 through the southern part of the city, producing its characteristic plateau. A Similar wide plateau 

 is conspicuously brought out on the east side of the same river. On this stand the state asylums 

 for the blind and for the imbecile. Its higlit above the sea is from 1080 to 1090 feet. At Mr. 

 Doyle's quarry on the west side of the river, the top of the limerock is about 1080 feet. The rock 

 here is all faded to an ashen or drab color, both by the oxidation of the contained pyrite, and by 

 the further oxidation and hydration of the iron-protoxide of the original blue color. Thus the 

 aluminous portions become more finely cemented than in the blue rock as seen at Mr. Cromer's 

 quarry, though the bedding is split and broken more by the weathering. This faded rock is more 



*This carbonaceous l:iyer extends eastward into Goodhue comity. 



|Of the last a .specimen is to he seen in the collections of Carleton college, and through the favor of Prof. L. B. 

 Sperry a photographic copy has been furnished the survey. It will be described in the volume devoted to the paheon- 

 tology of the state. 



