MAHVI.ANJ) GEOLOGICAL SDKVKV '^'3 



not to he iiiciisurcd l)y the time stiiiidai-d alDiic, luit also l)y the condi- 

 tions of life, wlicthev constant or variable, to which the ruriiis may he 

 siihjeclcd. Tri'sistent conditions must of necessity have less effect than 

 those Avhieh are changing. A satisfactory correlation of- the strata in 

 districts so widely separated as the ^Middle Atlantic Coastal Plain and 

 the Gulf liegion can l)c at1aiii('(l dnly liy a jjrofx'r appreciation ol" tin- 

 bearing of these factors on faunal development. 



In the Middle Atlantic Slope the relatively homogeneous nature oC 

 the Eocene deposits is a characteristic feature, indicating that through- 

 out the period of deposition the conditions were undisturbed by import- 

 ant physical changes. The fact that the de])Osits are made up largely 

 of secondary materials shows in a general way that their accumulation 

 took place near a coast reached by few large sediment-bearing rivers, and 

 that at the same time the place of deposition was sufl&ciently removed 

 from the coast line to be unalfected by shore conditions. In the Gulf 

 Eegion, on the other hand, deposition was more rapid, since a great 

 quantity of detrital material was brought to the sea by the large streams 

 draining the interior of the continent. The bearing of these different 

 physical conditions upon the interpretation of the two areas and the 

 correlation of their deposits will be fully considered in the body of the 

 report. 



The materials of the Eocene of the Middle Atlantic Slope, which are 

 so largely glauconitic, are of no little interest in themselves outside of 

 their hearing upon the questions of correlation, since few areas afford 

 such extensive deposits of greensand. On that account alone they de- 

 serve special consideration, and a chapter will be devoted to this phase 

 of the subject. 



The description of new species of fossils is of little scientific impor- 

 tance to the geologist unless the object is something other than the 

 mere multiplication of ncAv forms, which has too often been the case in 

 such investigations. When, however, the work has in view the fullest 

 230ssible representation of a fauna or the clearing up of doubtful points 

 in the synonymy of already described species, as well as a more complete 

 knowledge of their geological and geographical ranges, it becomes of 

 the very greatest value, since one whole class of important criteria for 



