234 THE GEOLOGY OF MINNESOTA. 



[Brick and lime. 



mounds and tables, capping the St. Peter sandstone, very far isolated from 

 the main area of the Trenton. To suppose this oalcite is due to springs 

 caused by the green shales, a common phenomenon now in Fillmore 

 county, is to require the former existence of a considerable thickness of 

 strata, all over the region of Caledonia, and extending far enough north 

 and east to furnish drainage surface sufficient to maintain such springs. 

 This is not inconsistent with the history of geological changes, nor with 

 the lapse of time since the Trenton rocks were elevated to the condition of 

 dry land. The present existence of isolated patches of the Trenton, both 

 in Minnesota and Wisconsin, can only be explained on the theory that the 

 whole formation was once more largely spread in horizontal strata over those 

 states, than at present. Then an extension of the Trenton so as to embrace 

 in one sheet of layers these isolated patches, is no more than enough to 

 bring also the Hudson River and the Galena into the region of these calcite 

 masses. The present outlines, shape and position of the areas of the Tren- 

 ton, demonstrate that they are only the relics of once greater areas which 

 have been eroded and removed slowly, and left as they are because they 

 have been better protected against destructive agents. While Root river 

 has been excavating the gorge in which it runs, 500 feet deep and two 

 miles wide, the Trenton limestone, which at first may have extended as far 

 north as to Hokah, has been slowly receding under the operation ot 

 denudation and surface drainage. These calcite masses then are relics ot 

 pre-glacial time, and perhaps of early pre-glacial time, since the last glacial 

 epoch did not operate in Houston county so as to disturb the older 

 surface.* 



Brick. The loam everywhere is suitable for making brick, which are 

 uniformly red. The following establishments were seen : 



Stephen Robinson, Money Creek ; two miles south of the village. 



Fisher & Keller, Caledonia ; began in 1875 ; burnt three kilns, and sold at $8.00 per 

 thousand. 



Brick were formerly made at La Crescent. 



The Lutheran society, at Spring Grove, manufactured on the spot a fine 

 red brick from the loam taken out to make room for the foundations and 

 basement of their church edifice. 



Lime. The Trenton and the St. Lawrence furnish all the quicklime 



*See the first annual report, p. 47. 



