6 Schuchert and Barrett — Revised Geologic 



tions of which glimpses only are obtained in the fragments of 

 the geologic record ; or they may be due to shif tings of 

 faunas, or to geologically sudden migrations into the con- 

 tinental or interior seas from the permanent or outer oceanic 

 reservoirs, the continuous realms of marine organic evolution. 

 The fossil faunas from the oceans spread as fast as the sea 

 transgressed the land, and, for practical purposes in strati- 

 graphy, may be accepted as having appeared simultaneously 

 in widely separated places. 



In all faunas there are more or less large percentages of 

 persistent species. These static and irregularly evolving forms 

 cannot therefore be used as fossils determinative of limited 

 geologic time. Although on the one hand the localized 

 species are of the greatest value in the stratigraphy of small 

 areas, the new forms which attain wide dispersal are, on the 

 other hand, of most significance in correlating the time stages 

 in separated regions, for they are the progressives, the time 

 heralders, as distinguished from their variously conservative 

 associates. Therefore in the chronologic correlation of the 

 stratified rocks most dependence is put upon a few species, 

 known as " guide fossils," together with the collateral evidence 

 of associated forms. These guide fossils may be of any class 

 of organisms and may be represented by many or few individ- 

 uals. The more abundant they are in individuals, the greater 

 is their geographic distribution apt to be, and the more easily 

 do they mark a geologic formation. On the other hand, the 

 wider the geologic distribution of a guide fossil, the less can it 

 be depended upon for detailed chronology. 



Locally successive, but distinct geologic faunas derived from 

 the same oceanic realm usually have a more or less ancestral or 

 direct genetic relationship with one another. In some cases 

 they are the returning, slightly altered descendants of an older 

 fauna, in other words, "recurrent faunas." Therefore the 

 possibility of a " break " in sedimentation between such super- 

 posed faunas is easily overlooked and the time value of the 

 recurrent faunas underestimated. Or, two locally superposed 

 faunas may be totally dissimilar, not only in the species but 

 even in the majority of the genera, and yet the time break 

 between them be a comparatively short one, the reason for this 

 unlikeness being that the two faunas are transgressions from 

 different oceanic realms and have therefore had independent 

 ancestral developments. 



A geologic " period " begins as a time of quiet, following a 

 disturbance and uplift of the land. The time of quiet is 

 marked by the erosion of the land and the spread of shallow 

 seas. Waterways broaden and unite across a continent, only 

 to be drained and destroyed by the crustal unrest with conti- 



