274 F. W. SARDESON ^A CORROSION CONGLOMERATE 



also black or where the stain has been altered or weathered. The black 

 may be changed to the yellow color of limonite. Associated with the 

 corrosion conglomerates in the Galena-Trenton series at Saint Paul, 

 Minnesota, there are two zones of yellow oolitic limonite (see figure 1). 

 The largest grains of the limonite have the form of corrosion pebbles. 

 Grains of this limonite are found that are an inch wide and one-fourth 

 of an inch thick and are shaped, as said, like corrosion pebbles, while 

 small grains are round, oolitic. This limonite-oolite is a form of corro- 

 sion conglomerate, or, as it appears to me, it is a black corrosion's product 

 changed by the addition of ferruginous deposition. If this limonite is 

 considered as a corrosion conglomerate, then black color appears a less 

 dependable characteristic than is the form of the pebbles; also weather- 

 ing or leaching of the black coating on the pebbles of zone number 8, 

 here described, changes it to rusty brown. In such cases the pebbles look 

 very much like ferruginous concretions when seen broken in a solid ma- 

 trix, or when they are weathered free from a shaly matrix and their 

 color has faded they can be recognized only by their shape. 



SURFACE OF PEBBLES 



Pitted or jagged surface features would presumably not appear on 

 corroded pebbles if the limestones of which the pebbles consist were very 

 uniform in texture and composition; but if the original surfaces of such 

 limestone pieces were concave or angular they might remain more or less 

 concave and angular, just as the original form of lenses and lumps is re- 

 flected still in the conglomerate here described. If, for example, the sea- 

 floor should be brecciated by the shock of earthquakes, either alone or 

 combined with the drag of sea-weeds, such breccia might remain com- 

 paratively rhombic even when well corroded. In other words, corrosion 

 conglomerate may be expected to bear close resemblance to breccia in 

 some cases as it does to concretionary structure in others, such as in the 

 one here described. 



AMOUNT AND DISTRIBUTION 



In amount of deposit corrosion conglomerate may be expected to be 

 rather thin, widely scattered, as in this case. Its distribution may also 

 be irregular. The black pebbles of the Galena at Saint Paul, Minnesota, 

 sometimes lie in small heaps, as if thrown together, and again they are 

 very thinly scattered. They recur for 80 miles. A few stand on edge, 

 yot enough stand edgewise in this case to make an "edgewise conglom- 



