Lckkl.] WISCONSIN. 329 
PORTLAND CEMENT RESOURCES OF WISCONSIN. 
nil! HAITIAN AND SILURIAN LIMESTONES. 
The Ordovician and Silurian deposits in Wisconsin contain heavy 
and widely distributed beds of limestone. It is necessary, however, 
to warn the prospector that the chances for obtaining a well-located 
limestone sufficiently low in magnesia to be serviceable for this use 
teem to be very poor. Of the numerous analyses of Wisconsin lime- 
stones that have been examined by the Writer, only three show a 
limestone carrying- less than 30 per cent of magnesium carbonate. 
Low-magnesia limestones exist in the State, but the chances seem to 
be against securing a satisfactory deposit of such material. So far as 
known, the only fairly thick and extensive bodies of low-magnesia 
limestone in Wisconsin occur in the lead region in the upper part of 
pe Platteville limestone," heretofore referred to the Trenton series. 
Mr. E. (). ririch states that in the southwestern part of the State a 
generally thin and rather locally developer! bed of relatively pure 
limestone forms the top of the Platteville. The "glass rock." as this 
bed is called, is probably better developed at Mineral Point and 
Platteville, Wis., than anywhere else in the lead district. At these 
localities it is filled with a highly characteristic fauna of late Stones 
River age. The rocks deposited upon it — the Galena limestone — are 
Black River and Trenton in age. 
Farther east, as at Beloit, and thence northward to Escanaba, Mich., 
Ithe "glass rock" is not represented in the sections. The "upper 
bull" and "blue," however, occur continuously east and north of 
Janesville and Beloit, Wis., while they are wanting, at least locally, 
in the lead region. Both the "upper buff and blue" probably — the 
latter certainly — represent a horizon intermediate between the lowest 
Galena and the "glass rock." 
Grant /; has recently described this particular limestone as follows: 
The "glass rock " belongs near the top of the Trenton limestone. This term has 
ibeen applied to a number of varieties of limestone in this general horizon. It may 
be said that the name is used for very line grained, compact, and hard beds of lime- 
stone which occur near the top of the Trenton. This is about as accurate a general 
definition of the term as can be given. There is, however, one particular phase of this 
rock which may be called the typical glass rock and which is apparently the rock to 
which the name was first applied. This is a very fine grained, very compact limestone, 
which breaks with a conchoidal fracture and which when fresh is of a light-brown 
or chocolate color. On exposure to the air, however, this color changes to a bluish 
gray. This phase of the glass rock is found in many places throughout the western 
two-thirds of the lead and zinc district: it usually occurs in thin beds, and in some 
places, especially a*t Platteville, is an important building material, the normal school 
building at this place and one of the high school buildings being constructed almost 
aBain, H. F., Zine and lead deposits of northwestern Illinois: Bull. U. S. Geol. Survey So. 246, 1905, 
pp. 18-20. 
&Grant, U. S., Geol, Wisconsin, vol. 2, p. 681, 1877 
