LITTIOLOGY OF THE SERIES. 183 



few feet of tlio top sliows tlio rouiKlcMl ({uartz uraiiis l)iiiU out into clearly 

 detineil crystals hy the deposition in crystall(\orai)hic continuity of pure 

 transparent silica. Irving lirst studied this phenomenon in the sand- 

 stones of the nortlnvcstern states,* as well as that of quartzite-building, 

 which is beautifully exemplified in this same sandstone layer at Osceola, 

 Wisconsin, near the station buildings of the ]\[innea[)olis, Saint Paul and 

 Sault Ste. Marie railway. Thin layers are scattered through the rock, in 

 which the grains have not only been enlarged, but they have attached 

 themselves together into a very firm nongranular quartzite. As the 

 nearly vertical walls of sandstone are eroded, these (juartzite layers stand 

 out as shelves of rock, resisting atmospheric effects and disappearing 

 only when gravity breaks off pieces whose attachments are weakened 

 through frost and fractures. The quartz grains of these layers are 

 cemented together by the deposition of silica in crystallographic con- 

 tinuity, and the extreme cleanness of the surfaces of the original grains 

 makes it dilHcult to tell in most cases where the contact of the old and 

 new material lies. 



THE SHALES. 



The Upper Cambrian shales are but little known in the northwestern 

 states. In Wisconsin seams of shale are intercalated, i)articularly in the 

 basal portion of the series f (Saint Lawrence), with a varying amount of 

 aluminous impurity present throughout the rock. In Iowa shales seem 

 to be wanting:, the rock being firm and presenting bold and picturesque 

 bluffs along the valleys. In Minnesota the borings of artesian and deej) 

 wells disclose a frequent shaly condition of the Saint Lawrence. Field 

 explorations confirm the view that the Shakopee must once have been 

 an extensive series of shales and limestones. The Oneota was a tolerably 

 pure limestone and the Saint Lawrence was partly limestone and partly 

 shale, and of great thickness. Quarries, as at Shakopee and Merriam 

 Junction, Minnesota, carry pockets of a clayey shale, which appears 

 originally to have been quite regular and continuous strata, but now 

 are squeezed out and reduced in bulk, or by a process of replacement have 

 passed into an impure dolomite. The brecciated condition freijuently 

 seen may have a similar origin. The angular fragments of the brecciated 

 layers are from a finely t(ixtured rock and they show on their surfaces 

 as well as within their mass the marks of having been dissolved to a 

 considerable extent while the transformation from their original condi- 

 tion to their present one was taking place. 



Nodules are characteristic of the topmost layers of both tiie Shakopee 



• On Socondjiry RnlarKomonts of Mincnil Knigm«nts in Certain Roci<8, Irving and Van Hiae: 

 Bullptin no. 8, U. 8. Gool. Survey, 1884, p. 40, pi. il. 

 fChannberlin : Geology of Wisconsin, vol. i, 1883, p. 140. 



XXVI— niLi.. Gkol. Soc. Am., Vol. fl, 1894. 



