3~~ The American Geologist. November, i898 
elsewhere and it should not be expected therefore that the 
phenomena, which arc clearly shown here, will be found as 
well displayed in other places even though the conglomerates 
had been widely distributed. In a few instances, however, 
such, phenomena are described from other places, and quite 
probably they are not uncommon. 
Prof. T. C. Chamberlin described one of these erosion or 
corrosion zones which is seen at Platteville in southwestern 
Wisconsin. He writes "The upper surface of the typical glass 
rock near Platteville, shows a smooth eroded upper surface 
with an occasional fracture of the upper layer through which 
the subsequent deposit penetrated."* This occurrence has 
been seen by me recently, and it is found to be at the top of the 
Stictoporella bed and thus corresponds to the fourth corrosion 
zone at Minneapolis and Saint Paul. I have already described 
the Orthisina bed conglomerate from Kenyon, Minnesota (op. 
cit. p. 29), which is like that at Saint Paul. Also the oolitic 
or semioolitic deposits which have been seen from Saint Paul 
for 100 miles southeastward at various exposures have been 
before mentioned (op. cit. p. 28). These occurrences with 
possibly many others which have not yet been noted may 
[)rove the wide continuation of the several corrosion zones in 
the lower and middle Galena (Trenton) series. 
The remaining beds of the series, numbers 7, 8 and 9. 
forming the typical limestone called Galena limestone in geo- 
logical reports is altered to a vesicular structure with many 
concretions in nearly all known exposures, and it will not 
<:herefore be readily determinable whether the recurring cor- 
rosion zones continue upwards into these beds. Such may 
have existed similarly or on even smaller scale than in the 
lower beds. Here and there they might be discoverable in 
well preserved strata. Very often in fact the rough faced 
strata of the dolomytes and the irregular compressed or re- 
duced shale laminae, simulate the corrosion zones. But, 
again concretionary structures and limestone nodules from the 
disintegration of shaly limestones might be mistaken for the 
conglomerate above described when percolating water had 
removed the black stain of their surfaces. Therefore it can 
not be assumed that corrosion zones exist where they have 
*Geol. Wis. vol. 4, p. 413 and fig. 8. 
