176 HALL AND SARDESON — THE MAGNESlAN SEMES. 



surface, while the quarried stone generally shows a white color. There 

 are rusted interlaminations producing a streaked section of rusted and 

 white layers.* 



THE ''MADISON SANDSTONE." 



The Madison beds in the country about Madison, Wisconsin, are 35 

 feet thick and consist usually of pure white, frequently loose, sand over- 

 laid by brown and yellow firmer rock. The upper layers generally show 

 a slight calcareous admixture which locally increases to 10 or 15 per 

 cent of the rock. It then becomes a good building material and is not 

 very sharply defined from the limestone above. Near the village of Mid- 

 dleton the bulk of the sandstone consists of a light yellow, friable, fine 

 grained dolomitic sandstone, composed of rolled quartz grains embedded 

 in a crystalline dolomitic matrix, the sand being 63.4 per cent of the 



rock.f 



As a whole; this sandstone is coarse grained at the bottom, becoming 

 finer grained toward the top of the formation. It seems everywhere to 

 be characterized by a strong crossbedding, as instanced at Hastings, Min- 

 neopa falls, Rapidan, and particularly Osceola. At the last named place, 

 at an exposure near the railway station, crossbedding can be seen reach- 

 ing a thickness of from four to six feet. As a rule, this rock is a clean 

 white sand, the coloration seeming always to be a local phenomenon. At 

 Osceola in the fossiliferous beds the color is a reddish brown, which is 

 due to an infiltration of fine oxide possibly from the surface, since the 

 rock was laid bare by denudation of the overlying formations. 



The thickness of the Jordan varies considerably, ranging between 35 

 and 90 feet. At the typical locality N. H. Winchell gives the total as 

 51 feet.J The writers, however, have at this same locality noted only 

 35 or 40 feet. At Osceola about the same thickness has been observed. 

 At Rapidan, in the Minnesota river valley and Bear creek, near the 

 Mississippi river, the thickness varies from 70 feet to 90 feet ; at Lan- 

 sing, 70 feet can be measured, with perhaps an additional 15 feet of 

 sandstone belonging to the Jordan. Thus the formation thickens toward 

 the south. 



FAUNAL CHARACTERS. 



Touching the paleontology of this sandstone formation it must be said 

 that in most localities fossils cannot be found. They are most numerous 

 at Osceola, Wisconsin, of all the exposures visited by the writers. Large 

 numbers of gasteropods and trilobites are. found here. At Rapidan, 

 Minnesota, are some gasteropods. Scolithus tubes will reward search in 



*Geol. and Nat. Hist. Surv. Minn., Ann. Rep. 1873^ N. H. Winchell and S. P. Peckham, p. 149. 

 fR. D. Irving: Geology of Wisconsin, vol. ii, 1877, pp. 535, 542, and other places. 

 X Log. cit., p. 149. 



