398 HISTOKICAL GEOLOGY. 



Second. The beginning of the characteristics of an era is to be looked 

 for in the midst of a preceding era ; and the marks of the future coming out 

 to view are prophetic of that future. 



Third. The end of an era may come, either after the full culmination 

 of the idea or phase, or earlier, at the commencing prominence of a new and 

 grander phase in the history. It may be as ill-defined as the beginning, 

 although its prominent idea may stand out boldly to view. Thus the era of 

 Coal-plants was preceded by the occurrence of related plants far back in the 

 Devonian. The era of Mammals was foreshadowed by the appearance of 

 mammals long before, in the course of the Reptilian era. And the era of 

 Reptiles was prophesied in types that lived in the earlier Carboniferous era. 

 Such is system in all history. Nature has no sympathy with the art which 

 runs up walls to divide off her open fields. 



Fourth. Mere length of time, without culminating or characterizing events 

 beyond that of rock-making, is not a criterion of value in the subdividing 

 of geological history. 



CORRELATION OF THE RECORDS. 



The chronological order is that demanded, as in any history. The first 

 object is, accordingly, to ascertain which are equivalent strata, or those of the 

 same geological horizon, and where in the chronological succession each 

 stratum belongs. 



As even the shorter divisions of geological time have in general been of 

 very long duration, the equivalent or correlate strata of distant regions can- 

 not be known to be precisely synchronous in origin. A long time, measured 

 by thousands of years, may in fact have intervened between the commence- 

 ment of beds that are most alike in all those points by which age and equiv- 

 alency are determined. 



Huxley, in view of the impossibility of determining true synchronism, 

 proposed to designate by the term homotaxial (from the Greek 6/i.os, same, 

 and Ta^ts, order) those strata, in regions more or less widely separated, that 

 have apparently the same relative position in the geological series. 



Difficulties. — The following are some of the difficulties encountered in 

 attempts to ascertain the true chronological succession : — 



1. The stratified rocks of the globe include an indefinite number of lime- 

 stones, sandstones, shales, and conglomerates ; and they occur horizontal 

 and displaced ; conformable and unconformable ; part in America and part 

 in Europe, Asia, and Australia; here and there coming to view, but over 

 wide areas buried beneath soil and forests. 



Moreover, even the same bed often changes its character from a sandstone 

 to a shale, or from a shale to a limestone or a conglomerate, or again to a 

 sandstone, within a few miles or scores of miles, and sometimes within a few 

 rods; or, if it retains a uniform composition, it changes its color so as not to 

 be recognized by the mere appearance. In the United States, many a sand- 



