270 F. W. SARDESOX — A CORROSlOX COXGLOMERATE 



bluish gray in color, so that the corrosion pebbles, which are black, appear 

 in strong contra-st. The matrix is shale and shaly limestone, with some 

 tliin strata of compact limestone. It contains also many lenticular lime- 

 stone lamina? and many nodular or irregular pieces of limestone besides 

 the conglomeratic pebbles. Those lenticular and irregular limestones in 

 the shaly matrix appear to be the kind of materials out of which the 

 blackened pebbles were made. The limestone pieces have the bluish color 

 of the shaly matrix, and thus are different from the conglomeratic peb- 

 bles in not being black. They are distributed in zones throughout the 

 bed vertically, while the conglomerate is in a limited zone. Those bluish 

 limestone pieces are not at all encrusted by marine fossils, while the con- 

 glomeratic pebbles often are. In shape these bluish limestones and the 

 black pebbles are, however, much alike: both comprise also the same di- 

 versity of texture, and they contain the same species of fossils. That the 

 conglomeratic pebbles are merely corroded and blackened limestone pieces, 

 such as those still abundant in the shales of this bed, is further evident 

 from the fact that a few flat pieces of limestone, 2 or 3 feet wide, have 

 been discovered to have their upper side blackened and corroded like 

 those of the pebbles, while their lower side is that common to mere lime- 

 stone pieces — that is, they are half-made pebbles. The pebbles contain 

 fossils — Receptaculifes oweni Hall, Clitamhonites diversa Shaler, and 

 others — some of which occur only in this bed number 6, so that the con- 

 glomerate could not have come from the disruption of an older bed. 

 Finally, there is no noticeable change of fauna between the strata below 

 and above the conglomerate. It is strictly intraformational. 



In considering the conditions under which this conglomerate was 

 formed, (1) the origin of the limestone lenses and nodules in the shales, 

 and (2) the force that turned some of them over as pebbles to be cor- 

 roded on both sides need to be explained. 



ORIGIX OF THE LlilESTOXE PIECES IX THE SHALES 



The nodules and lenticular layers of limestone in the shales of bed 

 number 6 are the direct result of deposition of lumps and cakes of lime 

 on the sea-bottom. The entire Galena-Trenton series ha,s such deposits, 

 and evidence of their origin can be seen in other parts of it, as well as in 

 bed number 6. For example, limestone bed number 1 is made up partly 

 of shaly seams, but mainly of irregular patches of more and less calcare- 

 ous material.* This structure is the pseudobrecciation as described by 

 R. C. Wallace in the same limestone formations of the Ordovician in 



* For chemical analyses of these, see C. W. Hall : Bull. Minn. Acad. Nat Sciences, 

 -pi. 3, p. 120. 1889. 



