﻿1XX PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [May 1905, 



one system from those of aftother. Subsequent researches showed, 

 however, not only that many of these physical breaks were of 

 limited horizontal extent, but also that other important physical 

 breaks, which had escaped detection, existed in the hearts of many of 

 the systems as originally defined. We know, for instance, that im- 

 portant unconformities occur in our own country in the Ordovician, 

 Carboniferous, and Cretaceous rocks ; and. as our classification has 

 undergone gradual modifications in the light of more extended 

 knowledge, the importance attached to these physical breaks for 

 classificatory purposes has steadily diminished, although their local 

 value is, of course, still recognized. 



As the result of earth-movements, cleavage and other meta- 

 morphic effects are sometimes available to assist us in separating 

 the rocks into groups, yet these effects have on the whole but a 

 local distribution, and mere allusion to them will accordingly 

 suffice. 



(5) Changes in the Nature of the Organisms included 

 in the Strata. 



With the advance of our knowledge of the strata and their 

 contents, the importance of fossils has become more and more 

 generally recognized ; but, even now, geologists appear to be im- 

 pressed with their value in different degrees. The writings of 

 eminent men of science who had not received a definite geological 

 training have, at any rate until quite recently, been quoted to show 

 the danger of correlation of the strata of different areas by means 

 of their included fossils, and much has been made of apparent 

 anomalies of distribution, notwithstanding that most of these 

 apparently-anomalous cases have already been proved to have been 

 founded on stratigraphical or paleeontological errors. I have else- 

 where touched upon this question, but in any discussion concerning 

 the principles of chronological classification of the sediments it is 

 necessary to pay particular attention to the assistance which fossils 

 furnish to those who are engaged in the classification. 



The tracings of the fossils on the geograms are, like those made 

 by lithological variations, of a blurred character, for more than 

 one reason. 



In the first place, there are errors of identification, and,*apart 

 from this, difference of opinion among geologists as to which 

 characters are of specific value, and accordingly the vertical ranges 



