254 THE GEOLOGY OF MINNESOTA. 



[St. Lawrence limestone. 



stone in Winona county, that are distinguished by different lithology, though 

 these distinctions are not known to extend very widely, viz.* 



1. Brecciated and concretionary. 



2. Eegular dolomitic strata. 



Below these are beds of transition to the St. Croix sandstone, made up 

 of alternating calcareous and sandy layers, aggregating a thickness of nearly 

 fifty feet, which have generally been referred to the St. Croix formation. 

 They are included in the first turfed slope below the precipitous bluffs of 

 limestone. 



The upper, brecciated portion of the St. Lawrence varies somewhat in 

 thickness, sometimes reaching seventy or eighty feet, and is not separated 

 from the regular strata by a marked and sudden transition. Indeed, it ap- 

 parently occurs interruptedly in the same horizon, and then is comparable 

 to the sudden concretionary areas that swell out and obliterate the strata 

 in the other formations, particularly the Waterlime and Niagara in Ohio 

 and Michigan,! and to the tors that are found in the limestones of the Car- 

 boniferous at North Anston, England.:}: There seems to have been some 

 irregularity in the ocean's bed at the time of deposition, and perhaps some 

 widespread sudden undulations of level which so disturbed the sedimentary 

 deposits at the time of their formation that when consolidated they not 

 only show remarkable differences of composition but also of stratification 

 and texture. This brecciated condition forms the bold buttresses which in 

 many places are seen near the tops of the bluffs, forming their prominent 

 features. It is illustrated by fig. 7, which is a view near Homer. The rock 

 itself is siliceous as well as calcareous, the silica sometimes appearing in 

 the form of arenaceous patches, or drusy geodes, and sometimes in the form 

 of chert of different colors. In the fissures much calcite is occasionally found. 

 The dolomitic portions are sometimes exceedingly fine-grained, and some- 

 times open and spongy; in the latter case, when exposed to the weather, 

 giving origin to caverns and small openings on the surface. Although the 

 outward aspect of such rock is that of a breccia, yet the re-cementing was 

 not due merely to subsequent sedimentation, but certain chemical and con- 



"These distinctions, however, have been mentioned in Iowa by Prof. J. D. Whitney, Geology of Iow, 1858. Vol. 

 I. p. 333. 



fGeology of Ohio, Vol. II., pp. 230, 374; Vl. I., pp. 62&-631. 



^British commissioners' report on the selection of stone for the new houses of parliament, De la Beche and Smith. 



