NO. 12 



SMITHSONIAN EXPLORATIONS, I917 



21 



good examples of the siliceous variety were secured only after much 

 hard labor, owing to the ready disintegration of the rock on exposure. 

 Efiforts were finally successful, however, and there was also secured 

 a mass of the so-called "edgewise " conglomerate several feet in 

 diameter, which will well illustrate the phenomenon of intra forma- 

 tional conglomerate described a number of years ago by Secretary 

 Walcott. This last was obtained where the steeply dipping lower 

 Ordovician beds outcropped in such a manner that the desired material 

 could be blasted without fracturing. The mass obtained is shown at 



Fig. 20. — Steeply dipping Lower OniuMcicUi strata near Hagerstuwii, Aid., 

 composed of " edgewise " conglomerate. Photograph by Bassler. 



A in figure 20. Figure 21, about one-sixth natural size, shows this 

 peculiar structure and the reason for the name " edgewise " beds 

 applied to these strata. All of such conglomerates are the result of 

 ancient mud deposits of tidal flats becoming sim cracked when exposed 

 to the air. The dried edges of the sun-cracked areas become tossed 

 about by the wind and the fragments finally accumulate in layers 

 which ultimately are hardened into rock-like conglomerate. Con- 

 glomerates usually indicate the base of a formation, but this particu- 

 lar kind may occiu- at any place within a formation, whence Mr. Wal- 

 cott applied the specific name " intraformational " to them. 



