POTSDAM SANDSTONE. 261 



The sandstones are of two kinds, those consisting of the common 

 white, buff, yellow or orange quartzose sand, with more or less of cal- 

 careous and aluminous admixture, and those formed by a mingling of 

 the common quartzose sand with green particles of glauconite, and 

 altered forms of it, producing as a result various shades of gray, yel- 

 low, green and mottled sandstones. The component grains frequently 

 have but a slight cohesion, so that the mass is fittingly termed green 

 sand. 



The upper and lower limits of the Mendota group are scarcely de- 

 finable. It graduates above into the Madison sandstone, so as to make 

 it difficult to draw a line between the two, and below, the alternation 

 of calcareous and arenaceous rock make it equally difficult to say 

 where the series ends. 



A thickness of fifteen or twenty feet would include the greater por- 

 tion of the lime rock, while about sixty feet would be required to 

 cover the entire alternating series. This will be clearly seen by in- 

 specting the local sections subsequently given. 



This constitutes one of the objections to separating this from the 

 Potsdam sandstone series, and grouping it with the Lower Magnesian 

 series. The lower limit of the Magnesian limestone is in this region, 

 and elsewhere so far as I have personally observed, well defined, al- 

 though sandstone mingles with the formation above and calcareous 

 strata extend scores, if not hundreds, of feet below into the Potsdam 

 sandstone. Aside from other objections, there seems to me no good 

 reason for placing the dividing horizon at any other point than that 

 which has very generally been recognized by geologists, viz. : at the 

 top of the Madison beds. 



It is to be remarked, however, that in central Wisconsin where the 

 Mendota beds develop a greater thickness and purity of dolomitic 

 character as well as greater lithological similarity to the Lower Mag- 

 nesian limestone, they have heretofore been mistaken for that forma- 

 tion, and something of confusion introduced into the geology of this 

 horizon, which the distinct recognition of the Mendota beds by Prof. 

 Irving has removed. 



The paleo'iitolofjical evidence very strongly corroborates the view 

 here taken. "Without attempting an exhaustive discussion, it may be 

 remarked that the Mendota beds are undoubtedly the eastern equiv- 

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

 characterized by the presence of Dicellocephalus Jtfinnesotensis, D. 

 Pepinensis, Lingula aurora, L. mosia, and a few other species of lim- 

 ited horizontal distribution. The primordial aspect of this fauna is 

 unquestionable. The collections of this season have shown Lingula 



