460 E. 0. ULRICH— CORRELATION OF THE STRAND-LINE 



rounded and up to 5 feet in diameter. Finally, the sandstone for 10 to 

 15 feet above this contact was shown to be thinner-bedded and less silici- 

 fied than is the more massive sandstone beneath it. Evidently the two 

 belong to distinct formations. And thus we proved that an exposure of 

 sandstone which until then had always been regarded as belonging to a 

 single formation in reality contains adjoining parts of two unconform- 

 able formations. In confirmation of their age assignment, I may add 

 that the contact between the Dresbach and the Franconia sandstones is 

 generally unconformable in Wisconsin. 



UNCONFORMABLE CONTACT OF CAMBRIAN AND OZARKIAN SANDSTONES 

 NEAR MADISON, WISCONSIN 



The second problem of this kind was encountered in the vicinity of 

 Madison, Wisconsin, where certain geologists believed they had discovered 

 evidence that must wreck my recent determination of the relations of the 

 Saint Lawrence formation and the Jordan sandstone, on the one hand, to 

 the Mendota dolomite and the Madison sandstone on the other. For 

 many years these two pairs of formations, the former having been named 

 in Minnesota-, the latter in Wisconsin, were regarded as respectively 

 equivalent. Two years ago, however, I became satisfied of their distinct- 

 ness. In fact, the Cambro-Ozarkian boundary was drawn between them, 

 the Saint Lawrence and the Jordan being referred to the older system 

 and the Mendota and Madison to the Ozarkian. 



The evidence relied on by my friendly opponents occurs in two excellent 

 rock cuts, one to the north, the other to the south of the city of Madison. 

 The section in both is essentially the same, the top being formed by basal 

 Oneota, beneath which is the Madison sandstone, while under the lowest 

 beds seen in the cuts are shaly beds which I admitted to be of Saint 

 Lawrence age. Now, as the Jordan intervenes in Minnesota and western 

 Wisconsin between the Saint Lawrence and the Oneota, and the Madison 

 sandstone in these cuts appeared to hold similar relations to the same 

 formations, it seemed at first sight as though my contrary interpretation 

 must be in error. But, after all, appearances in this case proved decep- 

 tive — our final conclusion leaves the formations as they were arranged by 

 me two years ago. 



Briefly, it was conceded that the shaly beds under the sandstone in the 

 cuts belong in the Saint Lawrence, and that they do not represent the 

 2 5 -foot Mendota dolomite that outcrops in bluffs on Lake Mendota and 

 at other points in the area between the two cuts. Next, it was conceded 

 that all of the eight known outcrops of the true Mendota are practically 

 identical in lithologic characters and contained fossils, and that they 



