GEOLOGICAL IMPORTANCE OF SEDIMENTATION 453 



basins of the British Isles, Norway, and the Arctic islands 10 the 

 north. 



This mountain-building, taken into consideration with the restrict- 

 ed nature of the interior sea of eastern North America between the 

 Cincinnati axis and the eastern border, forms geographic conditions 

 which should favor the development of extensive deltas filling up 

 shallow sea,s and giving rise to the formation of subaerial deposits. 

 Turning to the strata themselves to find an answer to this suggestion, 

 one notes the sparingly fossiliferous character of the Catskill group 

 of the Upper Devonian and the fact that the few fossils found arc 

 those of fishes, Eurypterids (Stylonurus), and some fresh- water 

 lamellibranches (Amnigenia), suggesting that, occasionally at least, 

 subaerial deltas may have covered considerable regions, and should 

 be looked for by a critical study of textures and structures. Many 

 geologists would grant this possibility, though it has found but little 

 recognition in geological literature. 



That dilTerences of view among able living geologists may be 

 held upon the subject of the Paleozoic formations is indicated by 

 the fact that Willis, as a result of his prolonged and detailed studies 

 of the Appalachians, interprets the Tuscarora (Medina) sandstones 

 of Maryland as submarine coastal plain deposits,^ while more recently 

 Grabau, in a preliminary paper, advances the hypothesis that the 

 Siluric conglomerates and sandstones are part of a huge subaerial 

 fan, whose apex was in southeastern Pennsylvania.^ 



This is not mentioned with the intention of urging the continental 

 point of view, since the cleanly sorted character of much of the forma- 

 tion would seem to indicate to the present writer that prolonged sort- 

 ing by waves, rather than the limited sorting and variable character 

 of river work, had been concerned. It might well be, however, that 

 an alternation of conditions has occurred, marine deposition dominat- 

 ing in one district, river work in another. 



Of the Paleozoic formations it is in the coal-measures, however, 

 that the relations between continental and marine deposits are most 

 distinctly shown and most fully appreciated; largely because the 

 climatic conditions were such as to lead to the formation of swamp 



1 Op. cit. (1902), pp. 55, 56. 



2 "Physical Characters and History of Some New York Formations," Science, 

 New Series, Vol. XXIT (1905), p. 533. 



