240 WAITMAN CROSS 
result of “this condition it ‘had been) decided) by si 1G. 
Gilbert, his predecessor in charge of the work, to establish the 
cartographic units of each area mapped upon the lithologic 
characters there displayed. But, as a matter of fact, the section 
in question contained fossils, and the present excellent represen- 
tation of the geologic development of the southern Appalachi- 
ans, afforded by the combined atlas sheets already published, is 
due in large degree to the use by Mr. Willis’s successors of fos- 
sils as guides in discriminating proper geologic formations 
where lithologic characters failed. By the use of fossils 
important stratigraphic horizons were recognized which had been 
entirely overlooked so long as local lithologic features were 
regarded as the only practical means of establishing carto- 
graphic units. 
The general chronologic scheme of geology has been estab- 
lished to express the evolution of the known portion of the 
globe. Its foundation lies in the fossils of the rocks, their 
demonstrated distribution and the evolution they show. At 
times the paleontologist, engaged in the biological side of the 
study of fossils, has seemed to believe that the chronologic 
scheme of geology was established to express the evolution of 
life upon the earth rather than the evolution of the earth itself. 
Sometimes the geologist, engrossed in the problems of the ‘‘new 
geology,” appears to forget his own rights and to concede to the 
paleontologist of the narrow view mentioned the correctness of 
his claim. But in these later days, as the geologist and paleon- 
tologist work more and more in harmony, such limitations of 
view are certainly becoming rarer, and it is more generally 
recognized that where a common scheme of time divisions fails 
to adequately express the known facts of earth development and 
life evolution some new provision is necessary. The ideas upon 
this subject recently advanced by H.S. Williams* seem to the 
writer to contribute much toward the adjustment of these 
difheulties. But it is) not) desired at this timelto discuss the 
«The Discrimination of Time-values in Geology,” Jour. GEOL., Vol. IX, p. 570. 
