HISTORICAL GEOLOGY 97 



series of rocks first studied and defined by him cannot be 

 disputed." We have seen that Murchison had priority 

 of publication in his term Silurian over Sedgwick's Cam- 

 brian, but that in a complete presentation, both strati- 

 graphically and faunally, the former had years of prior 

 definition. What has even more weight is that geologists 

 nearly everywhere had accepted Murchison 's Silurian 

 system as founded upon the Lower and Upper Silurian 

 formations. A nomenclature once widely accepted is 

 almost impossible to dislodge. However, in regard to 

 the controversy it should not be forgotten that it was 

 only Murchison 's Loiver Silurian that was in conflict 

 with Sedgwick's Upper Cambrian. As for the rest of 

 the Cambrian, that was not involved in the controversy. 



Dana goes on to state that science may accept a name, 

 or not, according as it is, or is not, needed. In the prog- 

 ress of geology, he thought that the time had finally been 

 reached when the name Cambrian was a necessity, and 

 he included both Cambrian and Silurian in the geologi- 

 cal record. The ' * Silurian, ' ' however, included the Lower 

 and Upper Silurian not one system of rocks, but two. 



It is now twenty-seven years since Dana came to this 

 conclusion, at a time when it was believed that there was 

 more or less continuous deposition not only between the 

 formations of a system but between the systems them- 

 selves as well. To-day many geologists hold that in the 

 course of time the oceans pulsate back and forth over 

 the continents, and accordingly that the sequence of 

 marine sedimentation in most places must be much 

 broken, and to-day we know that the breaks or land inter- 

 vals in the marine record are most marked between the 

 eras, and shorter between all or at least most of the 

 periods. Furthermore, in North America, we have 

 learned that the breaks between the systems are most 

 marked in the interior of the continent and less so on or 

 toward its margins. 



Hardly any one now questions the fact of a long land 

 interval between the Lower Silurian and Upper Silurian 

 in England, and it is to Sedgwick's credit that he was the 

 first to point out this fact and also the presence of an 

 unconformity. It therefore follows that we cannot con- 

 tinue to use Silurian system in the sense proposed by 



