EVALUATION OF STRATTGRAPIIIC CRITERIA 421 



contrary case, however, still remains to be shown, — that other examples of 

 such red shales and green sandstones may not be marine. Fortunately, 

 an illustration from the Devonian of eastern North America shows that 

 this combination of colors is there approximately restricted to an ancient 

 terrestrial formation and the colors become more uniform and show a 

 greater deoxidation where the deposit becomes clearly' marine. Marine 

 action, therefore, tends to eliminate these variegated color relations. The 

 details are- as follows : 



Ancient example, the Catskill formation. — The Catskill formation of 

 Xew York and Pennsylvania consists typically of thick members of 

 poorly laminated red shales interbedded with olive green or gray sand- 

 stones. Some reddish argillaceous sandstones also occur. In the Catskill 

 Mountains of southeastern New York the formation has a thickness of 

 3,000 feet and thickens to twice that amount farther southwest in Penn- 

 sylvania. In the eastern outcrops it represents the whole of the Upper 

 Devonian, but in passing west the Catskill is seen to overlap on the 

 Chemung, and only the uppermost Catskill occurs in western Xew York 

 and Pennsylvania. The Catskill and Chemung occupy the same time 

 interval, but represent landward and seaward facies in the sedimenta- 

 tion, the Catskill conditions gradually advancing westward . This forma- 

 tion is very poor in fossils, and such as occur are partly if not wholly of 

 fresh or brackish water forms. In the Chemung grays and olive greens 

 are the dominating colors of the whole formation, shales and sandstones 

 alike, and marine faunas occur at many horizons. The variegated color- 

 incf of the Catskill is thus seen to be correlated with the absence of ma- 

 rine fossils and to disappear in this case where marine conditions clearly 

 prevail. The indications of mode of origin which have been published 

 hitherto are, however, mostly of indeterminate value, the chief one being 

 that regarding the contrast to the marine Chemung, which has just been 

 pointed out. The writer has, however, studied closely a number of sec- 

 tions from the Hudson to the Potomac River and has found scattered 

 proofs of subaerial exposure at the time of accumulation. These are 

 to be observed, however, only under exceptionally favorable conditions, 

 and it is clear from this that they imply a far wider existence of sub- 

 aferial conditions. It is not the place here to describe these evidences nor 

 to discuss the problem of the Catskill in detail, but it is to be noted that 

 mud cracks and root-marks were found near Cumberland, Maryland, 

 extending through about 120 feet of strata close to the base of the Cats- 

 kill. On the Schuylkill River is exposed the southeasternmost outcrop 

 of the Catskill of the Appalachian geosyncline. This is the outcrop 

 which lies nearest to the sources of the 'sediment, but that it was some 



