528 GEOLOGY OF CENTRAL WISCONSIN. 



500 feet as an extreme, instead of 800 to 1,000 feet. Into this error 

 lie appears to have been led by committing the graver one of sup- 

 posing that he had to do with the base of the sandstone whenever he 

 found it in contact with the crystalline rocks, losing sight altogether 

 of the great irregularities of the upper surface of the latter rocks, by 

 virtue of which they rise high into the upper parts of the sandstone 

 series. In cases like that exhibited at the Dalles of the St. Croix, 

 where the Copper-bearing rocks are seen to rise nearly perpendicularly 

 through many feet of the sandstone, he regarded the traversing rock 

 as " intrusive," or of later origin altogether. 



Hall's investigations in the Central Wisconsin region do not ap- 

 pear to have been any more extensive than those of Dr. Owen, to 

 whose descriptions of the stratigraphy of the series he adds little that 

 is new. As regards the fossils of the formation, however, he makes 

 a very important contribution, 1 giving a list to which little has been 

 added by the present survey, as far as Central Wisconsin is concerned, 

 and a grouping of the fossils into those characterizing the lower, mid- 

 dle and upper beds, which, in its general order, proves to be quite cor- 

 rect. He commits the same error as Dr. Owen, however, with regard 

 to the thickness of the formation, placing it at only 500 feet. 2 As a 

 result of this, his list of fossils from the lower beds must be assigned 

 to about the middle of the series, below which are full 500 feet, about 

 whose fossils, or lack of fossils, we know nothing at all. This may be 

 regarded as a point of some importance in comparing the Wisconsin 

 Primordial with that of other regions by the fossils contained. Whit- 

 ney, who reports on the Lead Region in Hall's volume on Wisconsin 

 Geology, follows the latter gentleman in his under-estimate of the 

 thickness of the Potsdam series. 



Of all of the earlier accounts of the geology of Central Wisconsin, 

 I have found that of Dr. Percival, who worked after Owen and before 

 Hall and Whitney, by far the most reliable. Dr. Percival published 

 two small annual reports, in pamphlet form, whilst geologist of the 

 state, in both of which he gives descriptions of the Wisconsin forma- 

 mations, whilst one of the two gives an account of a reconnoisance in 

 the Potsdam sandstone region of the heart of the state. He recog- 

 nizes distinctly the very great thickness of the formation, its lack of 

 uniform character, and the fact that much confusion had been caused 

 by the reference to the Lower Magnesian,by Dr. Owen and his assist- 

 ants, of several distinct limestone bands separated by sandstone strata, 

 and regarded by Percival as included in the Lower sandstone, this 



1 16th An. Rep. N. Y. State Cabinet of Kat. Hist. 

 a Report on the Geology of Wisconsin, Albany, 1862, p. 16. 



