314 PROCEEDINGS OF THE ACADEMY OF [1887. 



THE CLASSIFICATION OF THE POST-CRETACEOUS DEPOSITS * 

 BY PROF. ANGELO HEILPRIN. 



The point of first importance to determine is -uliether all the 

 deposits succeeding the Cretaceous jieriod belong to a single major 

 system, or, as is generally recognized, to two distinct systems, the 

 Tertiary and Quaternary of geologists. It will probably be con- 

 ceded by all geologists that the only rational scheme of chronologi- 

 cal classification is that which can be made to be of universal appli- 

 cation ; in other words, a system that applies equally to all countries. 

 In our present knowledge, but one such scheme of broad classifica- 

 tion is known that which is based upon the rise and fall of success- 

 ive faunas. Granting a nearly equivalent development of life^forms 

 for the greater portion, if not the whole, of the earth's surface a 

 condition which can now be satisfactorily demonstrated, or at least 

 demonstrated to a variation within narrow limits it will be mani- 

 fest that we have in this development a true guage of chronological 

 relationship, and one that must be fairly exact in its application. The 

 grouping of systems will then be a mere deduction from the time- 

 record made to correspond to certain well-defined or rounded-off pe- 

 riods, so to speak, of faunal development, whose existence is made 

 known to us through the dissimilarities of successive faunas. 



How much, or what amount of, dissimilarity is considered suffi- 

 cient to mark out distinct systems is a matter of little consequence 

 to the geologist, but where uniformity of classification is required, 

 naturally only equivalent terms, or terms of approximately equal 

 value, should be used. The greater number of the major geological 

 systems now recognized are delimited by faunal dissimilitudes of a 

 more or less definite measure, which is indicated in the ratio of trans- 

 gressional forms uniting to the system below and to that above. 

 The present classification admits in a general Avay for each system 

 a faunal peculiarity measured in its lowest terms by some 35 or 40 

 per cent, and this classification has fairly met the demands of geol- 

 ogists in its application to almost all the entire series of sedimentary 

 rock deposits. Remarkably enough, in the case of the post-Cretaceous 

 deposits, Avhose classification has been effected probably by greater 

 niceties of percentage divisions than that of any other series, this meas- 



* Amplification of a report prepared, by request, for the American Com- 

 mittee of the International Con2:ress of Geolotrists. 



