316 
IOWA CAMBRIC SUCCESSION 
he ever really saw the uppermost member, which we now know as 
the Shakopee formation. This last mentioned formation only 
Shumard seems to describe clearly, but without actually recog¬ 
nizing it as different.^^ 
When in later years the two great successions — the dolomitic 
and the arenaceous — came to be subdivided, and distinctive geo¬ 
graphic names were given to the several members, there was pro¬ 
vided a basis for critical stratigraphic discussion. In addition, the 
supposed paucity of fossils was largely removed, and delimitation 
of the faunal elements were sharply defined. 
Although the biotic lines lately drawn by Hall and Sardeson^^ 
are perhaps as exact as the lithologic boundaries recognized, the 
remarkable clearness of the faunal delimitation is doubtless not so 
fundamentally significant in a stratigraphic way as the authors 
mentioned would have us believe. Their conclusions admit of 
wholly different interpretation. Nevertheless the recently ob¬ 
tained results concerning the vertical ranges of the several groups 
of organic remains strongly corroborate the arguments of the 
Minnesota workers. 
Throughout Cambric time the Upper Mississippi region was 
manifestly either a land area or was occupied by shallow epi¬ 
continental seas. This tract was subject to constant oscillatory 
movement that did not depart either up or down very far from 
sea-level. Of the enormous thickness of Cambric beds displayed 
in the west, in the Canadian Rockies, sections measurable in 
miles rather than meters or feet, little relatively was represented 
in Iowa. Still, the 1300 feet of vertical section made the sequence 
one of the great successions of our State. All Sediments of Early 
Cambric age and perhaps one-half of those of Mid Cambric date 
were unrepresented. If any of them ever were present over the 
province they were early removed during long land periods. 
Although the present areal exposure of these rocks within the 
limits of Iowa is so restricted, the terranal sky determinations in 
adjoining states are clearly reflected in deep-well sections of our 
own state. In this respect the log of the Tipton deep-well is 
particularly exact and illuminating.^® It is not probable that any 
13 Geol. Surv. Wisconsin, Iowa and Minnesota, p. 682, 1852. 
14 Bull. Geol. 'Soc. America, Vol. VI, p. 167, 1895. 
15 Iowa Geol. Surv., Vol. XXI, p. 449, 1912. 
