978 THE JOURNAL OF GEOLOGY. 
regarding the resources of his county as such has more interest to the 
ordinary intelligent citizen, . . . . than a report onan area embracing 
probably parts of several counties, though that area lend itself more 
naturally to scientific investigation, because limited by natural geo- 
graphic features or distinguished by some peculiarities of geologic 
structure.” 
The first work of the Iowa geologists was necessarily in the nature 
of a general reconnoissance of the entire field. That work done, the 
energies of the Survey will hereafter be chiefly devoted to areal 
geology, the county in each case being the areal unit. The present 
volume contains reports on six counties: Allamakee, Linn, Van 
Buren, Keokuk, Mahaska and Montgomery. Reports on two counties, 
Lee and Des Moines, were included in Volume III. Geological maps 
of the several counties are published on a scale of one-half inch to the 
mile. 
Geology of Allamakee County.— The volume proper begins with the 
geology of Allamakee county, by Samuel Calvin. This county is 
somewhat unique among the counties of Iowa in that it lies almost 
wholly within the Driftless Area. Its topography therefore is such as 
would be produced by the chemical and mechanical effects of destruc- 
tive agents acting, for a somewhat limited period, on beds of lime- 
stones, sandstones and shales in a region standing from 600 to 700 feet 
above base level. The main streams have cut their channels to base 
level, and the dividing ridges have their sides deeply scarred and 
gashed by multitude of divaricating erosion channels indicative of 
greater or less progress in the work of bringing down the ridges to the 
same level. ‘The tops of the ridges are regarded as co-incident with 
an old peneplain, and the difference in altitude between this peneplain 
and the base level of the present streams measures the amount of ele- 
vation that the region has suffered since the original peneplain was 
completed. 
The geological formations of Allamakee county include the Saint 
Croix sandstones of Cambrian age, and the Oneota limestone, Saint 
Peter sandstone and Trenton and Galena limestones of the Lower 
Silurian. Overlying the indurated rocks are beds of residual clays and 
sands, while over nearly the whole county there is in addition a thin 
mantle of loess. A few erratic bowlders, probably overwash from the 
margin of the drift, are scattered over the southwestern part of the 
county. The report is embellished with a number of engravings that 
