EVALUATION OF STRATlGRAPHlC CRITERIA 441 



folded or tilted, and it is seldom that a bed can be studied in two direc- 

 tions to a sufficient extent to determine the relations of the gravel strata 

 to the direction of sedimentation. 



Statements have been made regarding the characteristic shapes of 

 river as contrasted to shore gravels. But these have been founded on 

 few observations and without an analysis of the factors which determine 

 the forms of the pebbles. Walther is certainly more safe in his conclu- 

 sion that no distinction in form has been shown to exist between river 

 and shore gravels.*^ 



The subject of criteria between gravels of marine and terrestrial origin 

 was considered by the present w^riter in a paper before the Geological 

 Society of America in December, 1908. The pressure of other work and 

 the extensive problem into which this subject developed have prevented 

 thus far the final preparation of that manuscript for publication. As 

 much of it, however, is nearly finished the subject will not be here redis- 

 cussed. In that paper "the problem was approached by studying the 

 effects of shore, as compared with subaerial, activities on the production, 

 transportation, and deposition of gravel. It was determined that the 

 truly terrestrial forces produce vastly more gravel, spread it far more 

 widely, and provide more opportunities for deposition than do the forces 

 of the littoral zone. Conglomerate formations, therefore, should be 

 dominantly of terrestrial origin. In order to determine, however, the 

 mode of origin of particular examples, definite criteria must be drawn 

 between the two classes. It was shown that the thickness was one of the 

 most important of these, marine conglomerates, except under local and 

 special circumstances, being limited to considerably less than 100 feet in 

 thickness; terrestrial conglomerates, on the other hand, being frequently 

 measured in hundreds and occasionally in thousands of feet. 



^'Attention was next turned to the significance of the intercalated non- 

 conglomeratic beds and the relations to the under- and over-lying forma- 

 tions, with the conclusion that the characteristics of the associated strata 

 are frequently of high supplemental value for determining the mode of 

 origin." ** Especially where the finer textured beds carry evidences of 

 terrestrial origin the argument is strong that the associated coarser beds" 

 are also terrestrial. Where finer beds carry marine fossils the contigu- 

 ous coarser beds are presumably in part if not wholly marine. Where, 

 however, marine shales or sandstone are intercalated between conglom- 

 erate beds which are a hundred feet or more in thickness it is to be ex- 

 pected that at least the middle part of the conglomerates are terrestrial. 



«Einleitung in die Geologie, ISO?), p. 757. 



"Bull. Geo!. Soc. America (abstract), vol. 20, p. 620. 



XXXII — Bull. Geol. Soc. Am.^ Vol. 28, 1011 



