222 THE GEOLOGY OF MINNESOTA. 



[St. Lawrence limestone. 



stone, in Wisconsin,* notably on N. W.^ sec. 22, T. 7, E, 4 W., but he has 

 referred them to the St. Peter sandstone. 



The St. Lawrence, limestone. This is the most important formation in 

 the county. It not only occupies a greater superficial area of outcrop than 

 any other, but it takes the most prominent part in causing the varied topo- 

 graphy of the county. It surmounts the St. Croix sandstone, an easily 

 eroded rock, into which the valleys are deeply and rapidly cut, and main- 

 tains a bold and sharp outline along their tops. It is the immediate cause 

 of a great many hills and ridges. It confronts the observer in every nook 

 and on every promontory, along the whole course of the Root river, and 

 down the Mississippi bluffs as far as the state line, and it is especially con- 

 spicuous in the little valleys that ascend from those streams and that often 

 are more rocky than the larger valleys. 



The thickness of the St. Lawrence, in Houston county, is about 200 feet, 

 though Prof. J. D. Whitney has reported it as 250 feet thick on the Upper 

 Iowa river.f It is a dolomitic, or magnesian limestone. Its layers, while 

 generally regular and useful as a building-stone, are also sometimes very 

 much brecciated, rendering it at once more firm but also more refractory. 

 Thi j feature pertains to its uppermost thirty or forty feet. It furnishes 

 more stone for building than all the other formations of the county 

 combined. It is of a light, lively color and endures the weather perfectly, 

 showing not the least change in the oldest buildings in which it has been 

 used. 



The bedding in the upper portion of this formation is apt to be dis- 

 turbed by cherty, or concretionary, masses, which on the weathering away 

 of the bluffs become detached and fall into the bottom of the valley, where 

 they lie long after the non-siliceous portions of the rock have dissolved and 

 disappeared. Such cherty lumps are often a foot, or even two or three 

 feet, in diameter. They are roughened by cavities opening on the surface, 

 by solution and removal of the calcareous parts, and by the natural open- 

 ings and pores they acquired in the act of formation.' They are the only 

 portions of the formation in which fossils have been found in Houston 

 county. These masses sometimes show surfaces of drusy quartz crystals, 



'Geology of Wisconsin, Vol. II. 1873-77, p. C7J. 

 tGeology of Iowa, Vol. I, p 333, 1858. 



