EVALUATION OF STRATIGRAPHIC CRITERIA 417 



formations which are dominantly black, as in coal formations, but unless 

 the transition can be traced is not a positive indication; as seen, for 

 example, in the Lower Barren Coal Measures, where at Pittsburg a 

 marine fauna may be observed in a limestone band 2 to 3 feet thick, 

 resting on red shale and succeeded by 1 foot of black shale, passing again 

 into red shale/* The evidence thus shows a time of red shale deposition, 

 of possible marine origin, intervening in the Pennsylvanian when even 

 the land muds gave normally a black shale formation. Taking the 

 contrary case, lenses of gray or black shale in a formation which is 

 dominantly red, such as the black shale bands in the Triassic rocks of 

 Connecticut, is a suggestive though not positive indication as to the sub- 

 aqueous accumulation of the dark bands, perhaps as swamp deposits if 

 black; as lacustrine deposits if gray or olive in tone and associated with 

 thin and regailar bedding. 



^Yith respect to very early geological times, such as the Keewatin and 

 Huronian, the absence of red is of doubtful significance, owing to the 

 unknown composition of the air of those early periods and its possible 

 ineffectiveness as an oxidizing agent. 



Turning to the sea deposits, the dominant color of bottom muds at the 

 present time, omitting the abyssal red clays, is seen to be blue or gray to 

 black. There are, however, relatively small areas of red muds off the 

 shores of certain tropical lands. In other localities, as the Eed Sea, 

 yellow muds occur. The dominance of blue muds corresponds with the 

 known deoxidizing influences of the sea bottom, but at other times the 

 oxidizing conditions which now give local areas of red muds may have 

 been widely prevalent. Consequently in other geologic periods the domi- 

 nant color effect may be reversed. In the Pennsylvanian, for example, 

 the bulk of both terrestrial and subaqueous shales are dark with carbon. 

 In the Clinton formation, on the contrary, the deposits of the shallow 

 sea are brilliant with ferric oxide. 



The influence of climate and iho kinds of bacterial or inorganic reac- 

 lions which it favors is therefore a factor of stronger control than condi- 

 tions of continental or marine deposition. It is only in mean climatic 

 states, such as the present, that the place of deposition exerts a domi- 

 nating influence on color. 



Special examples of complex color relations are discussed in the follow- 

 ing topics." 



VARIEGATED FORMATIONS 



Green shales and red sandstones — Recent example, the basin of eastern 

 Persia. — Variegated formations offer special ))roblem"; which may be 



" Seen in the "briUiant cut off" of the ronnsylvauia Railroad at East I/iberty, 



