THE CAMBRO-SILURIAN QUESTION. 523 
as was finally thought by the author of the name. The lime- 
stones of the series cover a very large part of the Ozark region, 
and it is quite probable that a very considerable proportion of 
them will be found to be Silurian not only in Missouri but in 
Arkansas. In the Batesville region Trenton fossils have been 
recognized by Williams,’ and beneath the strata containing them 
are Magnesian limestones and sandstones which are referred to 
the Calciferous. ‘Only the upper members, however, are such. 
The larger part remain to be studied. Below them and exposed 
to the north is a series which according to Branner pass into 
Missouri.”? Farther to the west in the Ouachita district the 
novaculite rocks are said to be largely Silurian. As evidence a 
number of Graptolites have been described and a few molluscan 
remains recognized. 
The nearest region presenting rocks of similar age and lith- 
ological characters, one which has been, moreover, thoroughly 
investigated and with which the Missouri strata are to be com- 
pared, isin northeastern Iowa and the adjacent portions of adjoin- 
ingstates. It is therefore the Cambro-Silurian section of the Upper 
Mississippi that must serve as a standard of comparison for the 
Missouri rocks under consideration, and with which detailed corre- 
lations must be made. This fact necessarily has great weight in all 
attempts to correlate the rocks of the two districts. In the 
absence of faunal evidence that was at all satisfactory; with so 
small a proportion, in the Mississippi valley, of the Silurian exist- 
ing below the Trenton which is an horizon clearly defined in all 
parts of the basin; with a thickness in Missouri of Magnesian 
and Saccharoidal sandstone below the Trenton nearly twice as 
great as in Iowa between that formation and the top of the Cam- 
brian; with the evidence of a marked line of unconformity at 
the base of the First sandstone ; and with a considerable sequence 
of limestones and sandstones beneath the physical break men- 
tioned, the evidence appeared at the time of the recent review of 
the geological formations of Missouri amply sufficient for regard- 
t Arkansas Geol. Sur., Ann. Rep., 1890, Vol. I, p. 112. Little Rock, 1891. 
2Tbid., p. 116. 
