442 RB. Irving—-Primordial and Canadian Rocks of Wisconsin. 
in his monograph on the Fauna of the Potsdam Sandstone,* is 
doubtless at the horizon of the Mendota limestone. The name 
Mendoia is given from the exposures on Lake Mendota in Dane 
ounty, where the rock was first recognized as a separate stra- 
inches in thickness. This oolite consists of rounded granules 
z's’ to z'7”’ in diameter, closely packed in a pulverulent siliceous 
matrix, the whole having a milky white color. The smaller 
granules are seen under the microscope to be single grains of 
limpid quartz, the larger ones being aggregations of quartz 
grains. This layer is astonishingly persistent, having been 
seen everywhere where the base of this limestone is exposed, 
and at points many miles apart The whole thickness of this 
portion of the formation is about 20 feet. (2) Very heavily 
and indistinctly bedded light buff limestones, breaking with 
a conchoidal fracture and earrying considerable chert in nod- 
ules, in thickness about 20 feet; (8) the upper 8 ew of the 
* 16th Annual Report, Regents of the University of the State of New York, 
on the condition of the State Cabinet of Natural History, Appendix D, 1863. 
