McGee — Southern Extension of Appomattox Formation. 41 



geologic formations are correlated by the physical method, the 

 geographic distribution of organisms and the relations between 

 organisms and environment during the geologic periods may be 

 determined with only less accuracy than the like conditions 

 are determined to-day. So, by homogenic correlation, the ebb 

 and flow in the many branches of the vital stream during long 

 past eons will be measured, homotaxis will blend with homogeny, 

 and the cosmogony which it is the province of geology to de- 

 velop will be refined and ennobled. 



By reason of its prevalence, its distinctive coloration ; and its 

 fairly homogeneous composition, the Appomattox formation is 

 the most conspicuous deposit of the Coastal Plain between the 

 Roanoke and the Mississippi ; over much of this vast area 

 it has been traced in thousands of exposures and extensively 

 connected by stratigraphic continuity, yet the observations 

 have been fully interpreted and the exposures finally correlated 

 by homogeny ; and it is largely through the application of this 

 method of study that the formation has been found to be a 

 well defined and indivisible structural unit, representing a 

 single clearly defined episode in the development of the conti- 

 nent. So distinctive is the orange-hued loam, and so definite 

 the history recorded within it, that it is destined to rank as a 

 great datum formation, from which the stratigraphy and geo- 

 logic history of the Coastal Plain must be reckoned downward 

 and backward as they are reckoned upward and forward from 

 the Potomac. 



Hitherto no geologist has been able to form a definite con- 

 ception of the physiography of the southeastern part of the 

 continent during any given period. The episodes were too 

 short and the distances too long to permit satisfactory paleon- 

 tologic correlation ; the deposits vary from state to state, and 

 from Gulf province to Atlantic province; beds come in and 

 beds run out ; limestones change to shale and shales to sand- 

 stone ; with the changes in material there are changes in 

 fossils, and the complex history recorded in the everchanging 

 series has never been raveled; and not half the total thick- 

 ness of the beds yields faunae or florae in sufficient wealth 

 for close chronologic identification. Geologists have indeed 

 formed general conceptions of the development of the 

 province ; but they have been hazy in detail, shadowy with 

 respect to the succession of events, and vague with respect to 

 quantitative measures of time or deposit. But the Appomat- 

 tox formation, with the method of homogenic correlation 

 which it largely inspired, enables the student to represent 

 graphically and with fair accuracy the physiography of the 

 southeastern part of the continent during a well defined 

 episode. 



