SEQUEL TO SYSTEMATIC GEOLOGY. 581 



the known formations. And how far this gene- 

 rality and uniformity prevail, observation alone can 

 teach. The search for geological synonyms in dif- 

 ferent countries brings before us two questions ; 

 first, are there such synonyms? and only in the 

 second place, and as far as they occur, what are 

 they? 



In fact, it is found that although formations 

 which must be considered as geologically identical 

 (because otherwise no classification is possible,) do 

 extend over large regions, and pass from country to 

 country, their identity includes certain modifica- 

 tions; and the determination of the identity and 

 of the modifications are inseparably involved with 

 each other, and almost necessarily entangled with 

 theoretical considerations. And in two countries, 

 in which we find this modified coincidence, instead 

 of saying that the strata are identical, and that 

 their designations are synonyms, we may, with 

 more propriety, consider them as two correspond- 

 ing series ; of which the members of the one may 

 be treated as the Representatives or Equivalents of 

 the members of the other. 



This doctrine of Representatives or Equivalents 

 supposes that the geological phenomena in the two 

 countries have been the results of similar series of 

 events, which have, in some measure, coincided in 

 time and order; and thus, as we have said, refers us 

 to a theory. But yet, considered merely as a step 

 in classification, the comparison of the geological 



