Fig. 3S. 



TRENTON LIMESTONE. £91 



I. IJppEE Blue Beds, thickness, 15 feet. 



II. Upper Bvm Beds, thickness, 55 feet. 



LowEK Blue Beds, thickness, 25 feet. 



LowEE Buff Beds, thickness, 25 feet. 



It has been customary to divide the Trenton limestone of this state 

 into the " BufT limestone " and the " Blue limestone," and in the Lead 

 region the latter and some higher beds form the " Glass rock," "Brown 

 rock," and " Green rock." These latter terms are based upon litho- 

 logical characters that do not prevail in southeastern Wisconsin, and 

 hence the use of these terms would be undesirable even if their appli- 

 cation within the Lead region was sufficiently well defined to justify 

 their extension to other localities. 



To continue the unmodified use of the terms Blue and Butf lime- 

 stone, and to rest with this twofold subdivision, is open to serious 

 objection, as experience has shown. 



In the first place, the color distinction made between the two is 

 not applicable without qualification, since more than half of the up- 

 per portion usually designated Blue limestone is quite as persistently 

 bufi' as the lower division. The unleached interior of the thicker 

 beds, in all the subdivisions, is blue, and that was doubtless the origi- 

 nal color of the whole formation, but the two divisions designated in 

 this report as Buif are habitually leached to much greater depths 

 than the remaining two, and are less associated with bluish green 

 shales, which give to the latter a bluish or greenish aspect. Applied 

 as now suggested, the terms blue and buif become reasonably appro- 

 priate and very convenient. 



In the second place, the chemical distinction, viz. : that the lower 

 division is a dolomite, and the remainder a limestone, does not hold 

 good. An analysis of chippings from unweathered layers, represent- 

 ing the whole thickness of what has heretofore been called Buff lime- 



