180 H. S. WILLIAMS — SHIFTING OF FAUNAS 



The cross-lines connecting the sections, and marked from below up- 

 ward 1, 2, 3, 4, 5, are the limits of range of the several faunas in their 

 purity : 



The dotted line marked 1 represents the upper range of the fauna of the Onondaga 

 limestone ; 



2 represents the upper hmit of the pure Hamilton fauna. 



(I will use throughout the familiar name of the 

 formation, in which the fauna is typical as the name 

 of the fauna in each case.) 



3 is the lower limit of the Chemung fauna ; 



4, for the western sections, is the lower limit of the Waverly 

 faunas. In the Ithaca section {F) and the sections 

 farther east it is the highest level at which definite 

 traces of the Chemung fauna have been detected. 



5 is the base of the Glean conglomerate {E) and of other 

 conglomerates which have been regarded by strati- 

 graphers as its equivalent". In the east, at section I, 

 it is called Pottsville conglomerate series. 

 The line marked K represents first appearance of red sediments. 



Change in Thickness of Formations and its Meaning 



It will be observed that the total thickness of the columns increases 

 at an approximatel}^ uniform rate from west to east. The thickness from 

 the top of the Devonian limestone to the base of the Logan group in 

 central Ohio is 675 feet. The total thickness from the top of the Onon- 

 daga limestone to the base of the Pottsville conglomerate series in Monroe 

 county, Pennsylvania, is 11,300 feet, a gain of 16 times the thickness of 

 sediment in a distance of 500 miles. The increase of the Hamilton and 

 Marcellus of section C to the same formations in section I is from 445 to 

 2,200 feet, which is an increase of about fivefold. The increase for the 

 Avhole section between these two points is from 4,100 to 11,300, or about 

 threefold. The thickness of the sediments through which Genesee, 

 Portage, and Chemung fossils range in section A is 300 feet ; the range 

 of these same faunas in the section I is at least 4,000 feet. These com- 

 parisons will indicate that the diflference in thickness is due chiefly to 

 more rapid and greater accumulation of sediments at the eastern than 

 at the western ends of the line survej^ed, with an approximately regular 

 increase all along the line, and a continuance of the relative difference 

 throughout the time expressed by the total sedimentation. 



The evidence may be reasonably interpreted into the assumption that 

 during the time represented the chief source of the sediments was from 

 the eastward, and therefore that we ma}^ assume that the positions which 



