XO. 12 



SMITHSONIAN EXPLORATIONS, H)\/ 



21 



good examples of the siliceous variety were secured onlv after much 

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

 Ettorts 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 intraforma- 

 tional conglomerate described a number of years ago by Secretary 

 W'alcott. 'I'his last was obtained where the steei)ly dipping" lower 

 Ordovician beds outcropped in such a manner that the desired material 

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



Fii,. jij. — .Su-Lply (lii)i)ing Lower Ordovician strata near Hayerstowii. Md.. 

 composed of "edgewise" conglomerate. Photograph hy Basslor. 



A in figure 20. I'igurc 21. about one-sixth natural size, shows ihi^ 

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

 applied t(j these strata. All oi sucii conglomerates are the result of 

 ancient nuul depf)sits of tidal flats bec(jming sun cracked when exposed 

 to the air. Tlu- <lricd edges of tJK' sun-cracked areas become tossed 

 about by the wind and the fragments tinally accumulate in layers 

 which ultimate))" are hardened into rock-like conglomerate. Con- 

 glomeratcs usualh" in<licale the base of a formation, but this parliiu- 

 lar kind may occur at any place within a formalion. whence .Mr. W al 

 cott applied the specific name "" intra formalional " to them. 



