266 F. W. SARDESOX A CORROSIOX CONGLOMERATE 



interpreted ^^ corrosional in contT-ast with-sneh as are cammonly formed 

 by erosion. It is my purpose now to describe one of those conglomerates 

 in particular and to discuss its significance in relation to other con- 

 glomerates. The conglomerate to be described here is a thin or scattering 

 deposit in the form of black-surfaced pebbles of limestone (see plate 12) 

 in a greenish shalv limestone matrix. It occurs above the ba^e of the 

 Galena limestone formation proper. It is best known at Saint Paul, 

 Minnesota, but is seen also at Kenyon, near MantorYille, and in other 

 places where the base of the Galena is exposed. 



The mere occurrence of conglomerate in this region is not exceptional. 

 There are many conglomerate beds in the Upper Cambrian and the Ordo- 

 vician in the territory near Saint Croix and Mississippi rivers, in Minne- 

 sota and Wisconsin. Xearly all. if not all, of the several formations 

 comprised in the Cambrian and Ordovician here have some conglomerate 

 in them. The conglomeratic beds are, as a rule, thin deposits and, if 

 taken all together, they would not be very great in amount. They cor- 

 respond in that respect with the rather thin, verA- widely extended rock 

 formations in which they lie. They are all noteworthy as geologic evi- 

 dence, even though they are thin or inconspicuous. The bed of con- 

 glomerate here in the Galena limestone formation is even thinner or more 

 scattered than many others. On the other hand, its pebbles are remark- 

 ably well preserved and xeTv favorable for study. 



That which is considered to be most noteworthy about this particular 

 conglomerate is that it appears to be the result of corrosion, or at least 

 not of the ordinary- processes of erosion. Obviously, a widely different 

 conclusion results from the interpretation of a particular conglomerate 

 bed -as a marine corrosional product, as compared to that which follows 

 if it is interpreted as a basal erosional conglomerate. Whether the sea 

 made this conglomerate while relatively quiescent, or whether the sea 

 withdrew some hundred miles southward and returned again, leaving as 

 record of its changes these blackened cake-shaped jDcbbles, is the question 

 involved in case of the conglomerate here described. 



In regard to the interpretation of all conglomerates in this region, 

 they all lie in or adjacent to formations that bear marine fossils, and 

 where evidence to the contrary is not in preponderance they may, there- 

 fore, all be forthwith interpreted according to prevailing theories at the 

 present time, as evidence that the sea had- been repeatedly withdrawn 

 from the region and again returned to it. I may say here that I think 

 several of them may have been produced in such recession and readvance 

 of the sea, but that some others were not, and of these in particular the 



