NO. 12 



SMITHSONIAN EXPLORATIONS, I917 



23 



types of sediment, and the phenomenon of fossil coral reefs for 

 which the exhibit was primarily planned. 



The most valuable result of the summer's work was achieved at 

 Elkins, Kentucky. Here a single limestone slab, six feet long- and 

 several feet wide and thick, showing an unconformity distinct enough 

 to be appreciated by the layman, was quarried out and shipped to the 

 Museum without breakage, where it now forms a most instructive 



Fig. 22. — Unconformity between Early Black River limestone (white) and 

 Early Trenton strata (dark) at Elkins, Ky. Photograph by Bassler. 



exhibit. As shown in figure 22, the outcropping limestone ledge, sev- 

 eral feet in thickness, is composed of a distinctly white lower portion 

 and a dark-colored upper part, the head of the hammer marking their 

 line of contact. This line also marks an unusually clear unconformity. 

 Both of these layers are rich in fossils, those of Early Black River 

 (Lowville) age occurring in the lower white rock and those of Early 

 Trenton in the upper dark material. Since at other places in the 

 United States five hundred or more feet of strata of Middle and 

 Late Black River age intervene between these two layers, it is shown 



