THE ELEMENTS OF A PALEOGEOGRAPHIC PROBLEM 3 



Chicago), 1 river channels cut in an older formation,* etc. Such accidents 

 would bring in extrinsic material, both inorganic and organic. 



(a) ISOLATION" OF THE UNIT BY THE NATURE OF THE DEPOSITS. 



The most obvious character by which a bed may be isolated is the source 

 of the deposits, which is generally revealed in a broad way by the nature of 

 the materials and the included fauna or flora. Aside from the more common 

 criteria given in the text-books the following points may be noted : 



MARINE DEPOSITS. 

 COASTAL DEPOSITS. 



These may be either conglomerates or sandstones, resulting from the 

 action of waves in the advance or retreat of a strand-line. Advancing waves 

 will produce different results, dependent upon the character of the land 

 over which they make their way. If they are cutting back a high cliff of 

 hard rock, abundant bowlders and pebbles will be formed at the foot of the 

 cliff, to be later converted into a conglomerate, and if the cutting down of 

 the cliff is not completed the fallen blocks may be detected and identified 

 within the persistent mass at the foot of the former cliff, as in the sandstones 

 surrounding the Baraboo Range in Wisconsin. 3 This is a most significant 

 occurrence, as it locates the position of the strand-line in one interval of 

 time and determines one edge of the unit under consideration. 



A conglomerate formed by a sea encroaching upon a cliff composed of 

 strata of several geological periods would contain bowlders of different 

 kinds of rock bearing very different faunae. The age of such a conglomerate 

 could only be determined by its stratigraphic position ; it would not be at all 

 likely that fossil remains of contemporaneous animals would be preserved 

 in the conglomerate, because the shells would be triturated by the moving 

 stones before they were cemented into the conglomerate. Even if the cliff 

 were formed of rocks of a limited period of time, the resulting conglomerate 

 would contain fragments from numerous zones of life, so that a careful 

 analysis would be impossible. If such a condition is suspected the con- 

 glomerate should be traced, if possible, to its source and a careful study 

 made of the physiographic conditions when the bowlders of the conglomerate 

 were formed. The pebbles of a conglomerate should always be under 

 suspicion and the fauna of the pebbles never considered as representing 

 one formation or zone unless analysis of the source fully justifies such a con- 

 clusion. An admirable example of the mixture of faunae must occur in 



1 XVeller, Stuart, Journal of Geology, vol. vii, p. 483, 1899. 



1 Case, E. C., Carnegie Inst. Wash. Pub. No. 207, pp. 77 and 78, 1914. 



* Salisbury, R. D., and W. A. Atwood. The Geography of the Region About Devils Lake 



and the Dells of Wisconsin. Bull, v., Wisconsin Geological and Natural History 



Survey, p. 29, 1900. 



