ECKEL.] IOWA. 149 
It vj evident that materials suitable for the manufacture of cement 
are available, and this conclusion is confirmed by the fact that at 
Yankton, S. Dak., a plant has for many years been in operation in 
which similar beds belonging to the Niobrara are used. Further- 
more, in tests carried on at Sioux City cement has been made experi- 
mentally from the local material. a 
LIMESTONES. 
Nonmagnesian limestones are found in Iowa in the Ordovician, 
Devonian, and Carboniferous. The limestones of the Cambrian and 
Silurian are without important exception highly magnesian. Those 
of the Ordovician are predominantly magnesian, though an exception 
occurs in the case of the beds which it has been customary to map and 
discuss under the name Trenton. In eastern Iowa the dolomites and 
magnesian limestones have heretofore attracted more attention than 
the nonmagnesian rocks, and flourishing lime and building stone 
industries have been founded upon them. Limestone of one class or 
the other occurs in all of the eastern and most of the southern coun- 
ties. In the northwest the covering of Cretaceous and Pleistocene 
deposits limits the outcrops to a few deep stream valleys. The general 
distribution of the geologic formations of the State is shown on 
PL V. For details of localites the reader is referred to the various 
count} 7 reports of the Iowa Geological Survey cited in this text. The 
transportation facilities available at each point may be best learned 
from the large map of the State published and distributed gratuitously 
bj T the railway commissioners. 
ORDOVICIAN LIMESTONES. 
DISTRIBUTION. 
Below the Devonian but one limestone outcrops in Iowa which is at 
all suitable for Portland cement manufacture. It is known as the 
Trenton, and occupies portions of Dubuque, Clayton, Fayette, 
Winneshiek and Allamakee counties/' Under this name has been 
mapped an aggregate of nonmagnesian limestones and thin shales, 
[Varying in thickness from 15 to 350 feet. The variation in thickness 
is an expression of the fact that the difference between the Galena and 
Trenton is lithologic and not formational. It is probable that in the 
future the division will be made upon some other basis, but for present 
purposes the lithologic difference is the important one. The strata 
included on this basis within the Trenton are in the main either non- 
n Lonsdale, E. II., Proc. Iowa Acad. Sci., vol. 2, 1895, p. 173. 
'•Reports on the geology of Fayette, Winneshiek, and Clayton counties are now in preparation. 
For the geology of Allamakee County see Iowa Geol. Survey, vol. 4, pp. 35-120; for Dubuque County 
■ ibid, vol. 10, pp. 379-651. 
