584 



SCIENCE. 



[X. S. Vol. XXI. No. 537. 



not be taken as proof of the exact contem- 

 poraneity of two beds, and dissimilarity of 

 fossils within certain limits is not proof of 

 difference in age. 



When a littoral deposit is formed along the 

 margin of a transgressing sea its base at one 

 locality will not be syiachronous with its base 

 at another locality 100 miles farther inland. 

 The difference in date will be measured by 

 the time required for the sea to travel 100 

 miles across a subsiding area. In such cases 

 physical continuity of a foraiation is not 

 proof of exact contemporaneitj'. 



The facts are not questioned and the im- 

 portance of considering them in correlation 

 and time determinations is obvious, but it is 

 equally important to interpret them correctly 

 and not to exaggerate their quantitative value. 

 If undue importance is given to the theoretical 

 errors in time determination and correlation 

 caused by shifting faunas and transgressing 

 formations, there is danger of overlooking the 

 determinations that can actually be made or 

 of exciting unnecessary doubts concerning 

 them. These errors as a rule are geologically 

 trivial and scarcely measurable in terms of 

 the large units that must be used. The de- 

 gree of accuracy that is required in human 

 history is not to be expected in geologic his- 

 tory in which the time unit is so large and 

 the scale so coarse that an interval of a few 

 centuries is often not appreciable. The ac- 

 curacy of geologic correlation, so far as the 

 idea of contemporaneity is concerned, is com- 

 mensurate only with the nature of the rough 

 scale that must be used, and varies with the 

 completeness and the character of the evi- 

 dence in each special case. In general, the 

 farther a correlation is carried the broader 

 must be the terms in which it is expressed, 

 but horizons vary greatly in this respect. At 

 irregular intervals throughout the geologic 

 column there are limited zones characterized 

 by species or groups of species that have almost 

 world-wide distribution. These zones, as 

 Professor J. P. Smith has well stated, record 

 times of readjustment of faunal provinces 

 when for some reason interregional migra- 

 tions were made easy. They may, therefore, 

 be treated as geologically coiutemporaneous 



wherever they are found and they serve as 

 the solid frame-work of the general chrono- 

 logic structure. 



Barriers, migrating faunas and shifting 

 shores, make the local problems of correlation 

 more difficult in si>ecial cases. With insuffi- 

 cient data or insufficient experience, or both, 

 the geologist will make many mistakes, but 

 when all the obtainable facts are fully studied 

 from the broadest geological standpoint, when 

 every element of the fauna and flora is given 

 due weight, and when this is tested and sup- 

 plemented by all other available classes of evi- 

 dence, he can not only make correlations, but 

 he can determine contemporaneity within the 

 limits of accuracy that the subject demands. 



Dr. Wm. H. Dall spoke on the evidences 

 afforded by recent faunas, as bearing on the 

 notion of contemporaneity in fossil horizons, 

 drawing attention to the differences in num- 

 ber of species contained in a fauna relatively 

 to its latitude, varying from 180 at Green- 

 land, to 818 in the equatorial regions, a sub- 

 ject fully discussed in the U. S. Geological 

 Survey Bulletin ISTo. 84 ; to the total, or nearly 

 total, differences between the faunas of rocky, 

 sandy and muddy shores, due not only to the 

 different lithologic situs, but to the differences 

 of food-supply each afforded; to the fact that 

 to define a fauna in the paleontologic or bio- 

 logic sense, our definition must be wide enough 

 to include all these purely local variations, 

 and represent the whole population of a coast 

 with all its variation of conditions; to the 

 rapid spread of prepotent species given ac- 

 ceptable conditions, as in the case of Mya 

 arenaria, introduced on the Pacific coast, and 

 Litorina litorea on the Atlantic coast of the 

 United States, both of which are known to 

 have extended in profuse numbers over hun- 

 dreds of miles of coast where they were pre- 

 viously unknown, in the course of a few years. 



Dr. Dall also stated that the confusion of 

 ideas with consequent controversies which 

 have frequently occurred in connection with 

 these questions, are largely due to the attempts 

 on the part of geologists to combine in one 

 expression two irrelevant factors, the ' forma- 

 tion,' considered as a lithologic unit, and the 

 fauna as a time scale. A reference to the 



