198 
The American Geologist. 
April, 1896 
the combined thickness of the Galena and Trenton is quite 
uniform and that it does not vary much from 400 feet. In a 
general way the places where the observed thickness of the 
Trenton is least correspond to localities in which the Galena 
attains its maximum development. It is probable, therefore, 
that the two formations together represent a single strati¬ 
graphic unit and that the conditions favorable to the forma¬ 
tion of dolomite did not appear simultaneously over the area 
of sedimentation. In certain favored centers these conditions 
were introduced early, in others they were delayed till near 
the close of the epoch represented by the two formations com¬ 
bined. In certain localities the Trenton is only from 50 to 
100 feet in thickness: in others 350 feet of sediments accu¬ 
mulated before the process of forming dolomite began. The 
thickness of the Galena limestone, so far as it is exposed in 
Howard and Winneshiek counties, and in the vicinity of Post- 
ville in Allamakee county, is less than fifty feet, while the 
shales and limestones, below the Galena and above the Saint 
Peter, attain a maximum thickness of about 350 feet. Between 
Postville and the mouth of the Yellow river the Trenton is 
very much thinner than in the western part of Allamakee 
county, and the thickness of the Galena is correspondingly 
increased: the increase apparently taking place partly at the 
expense of number 4 and partly at the expense of the shales 
and shaly limestones overlying that member of the section. 
In the Postville well the shales in the upper part of the 
Trenton were reached at a depth of eighty-five feet from the 
surface. Three hundred and fifty feet lower the drill entered 
a sandstone at a depth from the surface of 435 feet. The 
position of the sandstone and the clean, limpid, water-worn and 
polished grains of quartz sand brought up by the slush bucket 
left no possible doubt that the Saint Peter had been reached, 
but for some reasons it seemed to be to the interest of certain 
persons to deny that the sand came from the Saint Peter 
sandstone. Boring proceeded, and at 448 feet from the sur¬ 
face the samples obtained showed practically pure Saint Peter 
sand. At 452 feet the most interesting anomaly which this 
interesting well presents was encountered. The sandstone 
suddenly gave place to what the well driller calls a mud rock 
and the progress of the work proved the “mud rock’’ to have 
