332 THE CAMBRIAN. 



(BULL. 81 



The upper anddower limits of the Menciota group are scarcely defin- 

 able. It graduates above into Madison sandstone, so as to make it 

 difficult to draw a line between the two, and below the alteration of 

 calcareous and arenaceous rock make it equally difficult to say where 

 the series ends. The Mendota beds are regarded as the eastern equiv- 

 alents of Dr. Oweu's Fifth Trilobite bed, the common horizon being 

 characterized by the presence of Dikelocephalus minnesotensis, D.pepin- 

 ensis,Lingula aurora, L. mosia, and a few other species of limited hori- 

 zontal distribution. The lower divisions constitute the main body of the 

 Potsdam sandstone and present but very few exposures in this region. 

 They are divided into four parts. 1 



The strata referred to the Potsdam sandstone by Prof. R. D. Irving 

 in central Wisconsin have a surface distribution of over G,000 square 

 miles. 2 In the vicinity of Madison the following succession of layers 

 between the Mendota base and the Archean occurs : 3 



Feet. 



1. Greensand layer 1 



2. Calcareous and dolomitic, friable, fine-grained, greenish sand- 



stone 31 



3. Not known 31 



4. Light colored sandstone, for the most part purely siliceous, 



being made of rolled quartz grains; but no specimens ob- 

 tained from the uppermost layers 704 



5. Red shale. 10 



Total 777 



The generalized section given above for the Potsdam series, below the Mendota 

 base, holds true for a large part of the central Wisconsin district, aud would be sat- 

 isfactory for all of it but for the facts next to be stated. 4 



Prof. Irving then proceeds to describe layers of sandstone and bowl- 

 der-conglomerate occurring uear the pre-Oambrian quartzite, which 

 apparently occupy a higher position and extend downward across the 

 upper portion of the eastern Madison section. He explains this occur- 

 rence on the theory of the accumulation of these conglomerates and 

 sandstones near the shore line at a higher level than the sandstones 

 that accumulated offshore. 



The beds of passage between the Potsdam and. Lower Magnesian series include, as 

 already said, two well marked beds, 60 to 90 feet in combined thickness — the Men- 

 dota limestone and the Madison sandstone. 6 



Of the fauna in the rocks he says : 



To the list of fossils of the lower sandstone series given by Prof. Hall but little has 

 been added by the present survey, as far as central Wisconsin is concerned. * * * 

 The general grouping of upper, middle, and lower species appears to hold true as 

 regards the order, but his lower species must really be assigned to the middle of the 

 series, since its thickness is about twice as great as Mr. Hall supposed. 6 



1 Op. cit., pp. 261, 262. 



2 The Lower Silurian rocks. (Geology of central Wisconsin.) Geology of Wisconsin, survey of 

 1873-1879, vol. 2, 1877, p. 529. 



»Op.cit.,p.534. 4 Op. cit, p. 53G. 8 Op. cit., p. 542. 6 Op. cit., p. 545. 



