740 ARTHUR C. TROWBRIDGE 
by the waves of a slowly advancing sea. In either of the first two 
cases, streams were the agents of its formation, the cause of the 
flat only being different. If a structural plain, the flat was devel- 
oped by streams, because they flowed down the dip of the top of 
the hard dolomite, scouring off the softer shale. If the flat is a 
remnant of a true peneplain in its beginnings, it was developed by 
streams because they had reached grade, and the region had been 
developed to an early peneplain stage before uplift allowed dissec- 
tion in another cycle of erosion. 
After analysis of the various features of this plain, the idea 
of marine erosion seems to be untenable. On such an extensive 
plain, marine near-shore deposits would have been sure to be 
deposited, and no suggestions of such deposits are found on the 
surface. Under this hypothesis, also, the mounds and ridges 
standing on the plain must have been islands, headlands, peninsulas, 
etc., and their sides must have suffered erosion by the waves which 
made the plain; that is, their sides must be shorelines of erosion. 
As most of these elevations stand out boldly on the plain, far from 
the main streams and valleys of the present cycle of erosion, it 
must be considered that they are about as they were left by the 
agent or agents which made them. This being the case, evidence 
of the work of waves on their sides should be visible now. But in 
contrast to this, the slopes of these mounds and ridges show every 
evidence of stream work, and have very clearly not been eroded 
to their present shape by waves; especially is this true in the cen- 
tral and southern parts of the county where the uplands form the 
stream divides and have the dendritic arrangement characteristic 
of topographies developed by the work of streams. 
The Galena plain has all the main earmarks of an incipient 
peneplain after uplift and partial dissection, except that it follows 
the Galena dolomite with surprising closeness: (1) it consists of 
extensive flat areas lying between uplands and lowlands, and in 
such positions flat areas are not developed except for some special 
reason; (2) aside from recent dissection, it has about the flatness 
to be expected on a peneplain; (3) distinct erosional hills stand 
above it; (4) it slopes gently in the direction in which the present 
streams flow; but (5) it follows the top of the Galena formation 
