STRATIGRAPIIIC CLASSIFICATION STRUCTURAL CRITERIA 455 



stone. As a rule, the material is imperfectly stratified, and often, espe- 

 cially between two limestone formations or between a shale and a lime- 

 stone, it suggests agglomeration of loose surface matter into mud balls. 

 When the transgression is over an old land area it is often red; occa- 

 sionally it contains enough of iron to constitute a low-grade ore. At 

 other times yellowish and green patches may be noted, while more rarely 

 there is a suggestion of fossil black soil. In most cases this initial de- 

 posit consists mainly of wave-transported regolith, and its principal service 

 is to pave the way for succeeding regular deposition by filling the decom- 

 position and solution hollows in the old land surface now in course of 

 submergence. Obviously, then, the deposit may vary greatly in composi- 

 tion, and it is always irregular in thickness and distribution. At one 

 point it pinches out to a few inches or nothing; at another near by it 

 may thicken to 50 feet or more. Its composition depends largely, and 

 sometimes entirely, upon the character of the rocks exposed in adjacent 

 contributing lands. Sometimes, however, prevailing winds and other 

 climatic conditions, that more commonly affect drainage and deposition 

 may introduce relatively foreign constituents in considerable quantities. 



The Chattanooga shale formation in Tennessee and in Arkansas and 

 Missouri offers many interesting and instructive illustrations of overlap 

 phenomena, and especially such* as concern initial deposits. The features 

 of the Tennessee occurrences have been described in sufficient detail by 

 Hayes and Ulrich.^*^ Those observed in Arkansas and Missouri are even 

 more varied. In northern Arkansas the Sylamore sandstone, evidently a 

 beach deposit, presents the prevailing aspect of a sandy basal deposit. 

 This sandstone reseinl)les the much older St. Peter sandstone, and in fact 

 consists chiefly of the more or less cleanly washed soil of areas in which 

 bodies of the St. Peter were then exposed. Though usually less than 

 2 feet in thickness, the Sylamore expands to much greater thicknesses 

 in places where it fills depressions in the preceding land surface. Very 

 interesting accumulations are found, too, that seem to represent fillings 

 of late Devonian caverns in the underlying Ozarkian dolomites. Even 

 more remarkable are the vertical, subtubular solution channels seen in 

 the quarries near Clarksville, Missouri. These extended completely 

 through a Niagaran' dolomite before the Chattanooga submergence 

 reached this point and filled the tubes with wave-swept black land debris. 



When little or no coarse material, either native or foreign in deriva- 

 tion, is included in the initial deposits, the lowest bed or beds of the over- 

 lapping formation often resemble the underlying rock in composition. 



BO Geol. Atlas U. S., Columbia folio, No. 95. 



