464 E. O. ULRICH REVISION OP THE PALEOZOIC SYSTEMS 



as shown in figure D, page 450, the intercalated beds may themselves 

 bear unconformable relations to both the superposed and the underlying 

 stratigraphic units. 



The principal lithologic criteria employed in locating the line of 

 unconformity were mentioned in discussing the peculiarities of initial 

 deposits in relation to overlaps (see page 454). Here we need to em- 

 phasize only those bearing more directly on sea withdrawal. The most 

 common of these, and from that standpoint the most important, is the 

 red or rusty color that pertains so frequently, though perhaps but locally, 

 to deposits immediately following an unconformity. While practically 

 all red deposits are either directly or remotely related in their origin 

 to subaerial decomposition processes, and are thus connected with land 

 conditions, it does not by any means follow that all deposits of this 

 color are indicative of stratigraphic unconformities. On the contrary, 

 the larger accumulations of red sediments are not directly associated 

 with such structural phenomena. Often they begin considerable distances 

 above the recognized unconformity which, moreover, may show no red 

 deposits at all, even when a basal conglomerate is present. An instance 

 of this kind was observed in the Wichita Mountains of western Oklahoma, 

 where presumably Permo-Carboniferous "red beds" overlapped on early 

 Ordovician limestone. It is the thin beds of this color, say from an 

 inch or two to not over two feet in thickness, that separate two lime- 

 stones, or a calcareous shale from an overlying limestone, that are 

 especially significant in this connection. This layer is but rarely well 

 stratified, and usually it is softer than the rocks on either side. In 

 weathering it breaks down quickly, leaving a conspicuous, hollowed-oul 

 space. Very commonly, too, it is marked by a line of springs. In som* 

 3ases it seems probable that the rusty color is not original, but is to be 

 attributed to percolation of ferruginous waters. 



This basal layer, whether red or not, is often highly suggestive of 

 reworked soil. Soils preserved in situ, with erect trunks of trees still 

 rooted in them, have been described. Such preservation, however, in 

 the case of lands being submerged by a sea, could have occurred only 

 under exceptional circumstances. A single probable occurrence of this 

 kind has been observed in lower Paleozoic rocks; and even in this case 

 it seems to be only the deeper part of the subsoil that remained undis- 

 turbed; namely, in the vicinity of Eureka Springs, Arkansas, a layer is 

 sometimes seen at the contact between the Jefferson City dolomite and 

 the Yellville formation that seems to be a part of the residual mantle 

 recemented in dtu. It contains lenses of chert and silicified masses 

 of Cryptozoon minnesotense. Weathering on the old land surface, these 



