642 PRINCirLES OF STRATIGRAPHY 



certain. In the Alaskan case it may be due to the abundance of cold 

 water carried in from the land by the melting of the glaciers, which, 

 as shown by Tarr (92), has recently become very marked through 

 changes which also caused an advance of the glaciers in certain lo- 

 calities. Portions of the glaciers hitherto protected by debris or 

 otherwise have been suffering ablation during July and early 

 August at a rate sufficient to lower the ice surface four inches a 

 day. 



While marine fossils are as a rule a reliable indication of the 

 marine origin of a given series of elastics, this is the case only when 

 the fossils are generally distributed throughout the mass, or when 

 there is no other positive indication of a non-marine origin. As has 

 already been shown, eolian limestones composed almost wholly of 

 marine organisms are forming at the present time, and have formed 

 in the past. Fossils weathering out from a marine series may be 

 incorporated in the next overlying continental formation, as in the 

 case of the Eocenic and other fossils of the rocks forming the floor 

 of the Libyan desert, which are included in the overlying desert 

 sands. Marine organic remains may be carried inland by winds, by 

 birds or in some other manner, and thus become incorporated in 

 terrestrial formations. Finally, deposits of terrestrial character may 

 be at intervals submerged by a momentary encroachment of the sea, 

 as in the case of the Po, the Rhine and other deltas, with the result 

 that intercalated marine sediments are formed. Or, again, the sea 

 may invade a large territory previously the theater of terrestrial 

 deposition, and by reworking the upper layers of the continental de- 

 posits, or in some cases the entire mass, may impress upon it lo- 

 cally a marine character. This has been the case with the St. Peter 

 sandstone, largely an eolian formation ; with the upper part of the 

 Sylvania sandstone of similar origin ; with the Dakota and ap- 

 parently also with the Potsdam sandstone, which, in many sections, 

 still shows characters pointing to torrential or eolian origin of a 

 considerable portion of the rock. 



It should of course also be emphasized that the reverse is likely 

 to hold, namely, that absence of marine fossils is not an absolute in- 

 dication of the non-marine character of a formation, though absence 

 over a very large area may probably be taken as a fairly certain 

 guide. The physical characters of the rocks and their relationships 

 must be taken into careful consideration. Among the negative char- 

 acters of marine elastics are : the absence of mud cracks, rain prints, 

 footprint impressions, rill marks, etc., though all of these may 

 occur in the shore zone. 



