PRINCIPLES OF STRATIGRAPHiC CORRELATIONS 511 



continental sea itself. On resubmergence either the same oceanic basin 

 at once resumed its contribution of life, or oscillation may have changed 

 the geographic pattern so that the new sea drew its fauna from an al- 

 together different source. In the former case the life break may appear 

 insignificant; in the latter it is proportionately much greater than the 

 time break. The conditions referred to are at the basis of the following 

 principle.) 



(6) Varying time values of faunal breaks. — The relative magnitude of 

 a faunal break is ofteti ividehj disproportionate to the time break. One 

 may be either greater or smaller than the other when measured according 

 to an hypothetical standard. Thus a total change may be occasioned by 

 superposition of an Arctic or a Pacific fauna or a north Atlantic fauna 

 on a Gulf of Mexico fauna, and yet the time value of the break be rela- 

 tively insignificant. Of many examples I may cite the abrupt and total 

 change in faunas encountered in passing from the upper Stones River into 

 the basal, upper Chazy, member of the Chambersburg limestone near 

 Mercersburg, Pennsylvania, and from this again into the overlying Low- 

 ville member. That the time value of these breaks is not in due pro- 

 portion to the faunal change is shown by the much smaller differences 

 noted in comparing the faunas of the Stones River, which underlies the 

 upper Chazy, and that of the Lowville, which overlies it. ( See also pages 

 545 and 554.) As for the cause of the differences in value it is found in 

 the fact that whereas the upper Chazy fauna is north Atlantic in origin 

 the others migrated from the Gulf of Mexico. A case of like significance 

 is seen in the Curdsville limestone, which contains a large northern fauna 

 that is intercalated in the midst of very different southern faunas. 



On the other hand, the difference between two immediately superposed 

 faunas may appear insignificant, although the time break between the two 

 is really of very considerable magnitude. The slight difference between 

 the upper Stones River and Lowville faunas just mentioned serves well to 

 illustrate the idea. In fact, when, as frequently happens, these two also 

 lithologically similar formations are in contact it is often difficult to find 

 the plane of separation. And yet the hiatus, as shown in east Tennessee, 

 represents pure limestone, shale and sandstone deposition aggregating 

 thousands of feet in thickness. (See pages 554 to 557.) Tt is well illus- 

 trated also by the stratigraphic relations of the closely simulating Catheys 

 and Leiper faunas in middle Tennessee, where the two commonly are in 

 contact and require great care in their separation. However, in central 

 Kentucky and in the vicinity of Cincinnati the Catheys formation is at" or 

 near the summit of the Mohawkian and is followed by 100 to 300 feet of 



