HOUSTON COUNTY. 223 



St. Croix sandstone.] 



also amethyst crystals, and great quantities of pyrite oxydized and hy- 

 drated so as to produce limonite, the form of the crystal alone remaining to 

 indicate the original mineral. A careful study of these fossils has not yet 

 been made. From Houston county have been obtained from such cherty 

 lumps, an Orthoceras resembling 0. primigenlum, H., but having an oval 

 section and oblique septa; an Orthoceras with septa nearly directly trans- 

 . verse to the direction of the shell, much more resembling 0. prim iff enium, H., 

 and several species of spiral univalves including some of OpMleta and some 

 of Pleurotomaria. 



The St. Croix sandstone. This name was applied in the first annual report 

 provisionally to the light-colored and often friable sandstones which occur 

 along the Mississippi river in Minnesota, and which have by some been 

 regarded as the stratigraphical equivalent of the Potsdam sandstone of New 

 York. This was done because, in the existence of another formation, of 

 different lithology, affirmed also to be the equivalent of the New York 

 Potsdam, it was necessary to have some designation for each of them. It 

 seemed, from considerations there given, that the lower of these two sand- 

 stones was the probable equivalent of that formation in New York. 



Since that report was published considerable more time and observation 

 have been given to the same question. Numerous facts from the northern 

 part of the state, where the lower of these two sandstones appears abun- 

 dantly, have been gathered, and some of them, with theoretical and min- 

 eralogical considerations, have been published in succeeding reports of the 

 progress of the survey.* They all go to affirm the essential correctness of 

 the distinction brought forward in the first annual report. Hence the 

 designation St. Croix sandstone is retained. The reasons in full for this can 

 not be given here. Meantime if, before the final discussion of this subject, 

 the reader desires further facts bearing on it, he is referred to the annual 

 reports, particularly to the ninth and tenth. 



Although these sandstone beds occupy the river bluffs along the Missis- 

 sippi and the Root rivers throughout the county, they afford but very few 

 opportunities for satisfactory examination. They are in the lowest part of 

 the bluffs and are generally hid by a sloping talus that is usually turfed 



Ninth and tenth annual reports. 



