182 THE CAMBRIAN. Twill. 81. 



light-colored sandstones that occupy a conspicuous place in the blnffs 

 of the Upper Mississippi, below the Lower Magnesian limestone. Of 

 the Sioux quartz ite he says that — 



Its features here are easily identifiable with those of the Potsdam at the rapids in - 

 the St. Mary's River, at Sanlt Ste. Marie, Michigan. In their passage to the west, the 

 over-lying, light-colored saudstones seem to become more largely developed. They 

 acquire a thickness, including the intercalated beds of shale, of about 600 feet in their 

 exposures along the Mississippi River. l 



Of the fossils collected from the horizontal beds at the Falls of the St. 

 Croix it is said they : 



Were described as coming from the Potsdam sandstone, and were supposed to belong 

 to a horizon much lower than that of the Lingula Beds of the Potsdam of New 

 York. The name has been still further removed from its original use by the Iowa 

 geologists, in its application onljf to these upper beds, and in giving the name Sioux 

 quartzite to the western representative of .the original Potsdam. Dr. Owen, although 

 he recognized many points of difference between the Lake Superior and New 

 York Potsdam, and these light-colored sandstones of the St. Croix and Upper Missis- 

 sippi, seems not to have noted the important fact that the former are everywhere sub- 

 ject to distortions and fractures by volcanic forces, while the latter are never known 

 to be disturbed by such causes. It is true that he embraces both the red and the 

 light-colored sandstones in the designation of " Potsdam, " and argues at length to 

 prove the great age of the red. (Geological Survey of Wisconsin, Iowa and Minne- 

 sota, p. 187.) 



It is in accord with geological precedent, therefore, to separate these two sandstone 

 formations under different names, retaining the name of Potsdam for the older, and 

 giving provisionally the name of the St. Croix River, on which they are best exposed, 

 to the latter. 



The following reasons may be assigned: 



(1) The Potsdam beds were laid down before the close of the volcanic disturbance so 

 evident in the rocks of the early Silurian and pre-Silurian ages; the St. Croix beds 

 were deposited and still lie in horizontal layers, u neon form ably not only over the 

 Laurentian and latest trappean rocks of the northwest, but also on the upturned 

 beds of the Potsdam. 2 



(£) This reason has little bearing and will not be quoted. 



(3) The lithological characters of the Potsdam beds are uniformly different from 

 those of the St. Croix beds. The former are hard and often vitreous, usually of a 

 brick-red color. Their bedding is very distinct, and often separated into slaty layers 

 by parti ngs of red shale. They are strongly marked by the so-called fucoidal im- 

 pressions. They are frequently ripple-marked and sun-cracked. The latter are 

 white or buff-colored, often friable, and constitute a heavy bedded or massive sand- 

 stone, of handsomely rounded quartzose grains. 



(4) This notes differences in chemical composition. 



(6) 3 The Potsdam sandstone has a thickness of at least 400 feet; (Note— D. Owen 

 makes the thickness of the Potsdam (red sandstones of Lake Superior) over 5,000 feet. 

 See Owen's report on Wisconsin, Iowa, and Minnesota, p. 193); tho/Stf. Croix sandstone 

 also has a thickness of over 500 feet. It is more in keeping with the canons of geo- 

 logical nomenclature to give separate titles to formations so well defined and so largely 

 developed. 



(7) This gives the evidence of paleontologic differences. He states 

 that the fossils of the Potsdam sandstone of New York are Lingula 



1 Op. cit., p. 60. * Op. cit., pp. 69, 70. ' 5th does oot occur iu the origiiial. 



