ulrich: major causes of oscillations 73 



of the lithologic and faunal sequence of the Cambrian in the 

 western half of the State, the most important improvement 

 brought about by the first season's work was the proof that the 

 Mendota Hmestone and Madison sandstone are really post- 

 Cambrian formations and not, as had been supposed previously, 

 the eastern representatives of, respectively, the St. Lawrence 

 limestone and the Jordan sandstone of Minnesota. In fact, it 

 was then believed and has since been definitely proved that 

 whereas the St. Lawrence extends uninterruptedly from Minne- 

 sota and Iowa across the southern half of Wisconsin and under 

 cover of later formations into northern Illinois, the Mendota 

 limestone is entirely absent to the west of a narrow trough run- 

 ning southeastwardly from the southern slope of the pre-Cam- 

 brain Baraboo quartzite range. 



In the following field season of 19 15 doubt arose as to the 

 eastward extension of the Franconia formation to and beyond 

 Madison. At this place there is a more or less decidedly cal- 

 careous sandstone formation, approximately 100 feet in thick- 

 ness, which lies between unquestionable Dresbach sandstone 

 and no less certainly estabhshed St. Lawrence limestone and 

 shale. The intervening formation therefore seems to occupy 

 the same stratigraphic position as the Franconia. But its 

 lithological characteristics, except that it also contains consid- 

 erable, though more disseminated glauconite, are quite dififer- 

 ent from those of the Franconia ; and whereas good fossil remains 

 of characteristic types are exceedingly abundant in the Fran- 

 conia they appear to be much fewer and, so far as could be de- 

 termined from the handful of fragments then procured, of differ- 

 ent species. 



In casting about for a means of determining the problem I 

 thought of an old anticline that extends southwestward from 

 the Baraboo range across southern Wisconsin into IlHnois. This 

 axis had previously been found to have had an important effect 

 on the distribution of the Ordovician formations and it seemed 

 worth while to see whether it had not been in existence, and func- 

 tioning as a barrier, already in the Cambrian. Accordingly, 

 a part of the season of 1916 was devoted to following the nearly 



