COMPARISON BETWEEN EUROPE AND NORTH AMERICA 559 



by widespread disconformities, and such breaks in sedimentation also 

 succeed, and probably very generally precede, the series. That these 

 breaks in the sedimentary record are due to extensive retreatal or negative 

 eustatic movements of the sea seems unquestionable, and on this account 

 it would appear that such breaks furnish valuable datum planes for de- 

 limiting definite divisions of the stratigraphic scale inclosed between 

 them. It must, however, always be borne in mind that the retreat of the 

 sea and its readvance occupied a considerable time interval, and that 

 during such movements sedimentation continued longer and recom- 

 menced earlier in the region of later emergence during retreat, and of 

 earlier submergence during readvance, of the sea — that is, in the region 

 farthest removed from the shoreline at the beginning of the retreatal 

 movement— than it did in this shoreline region. Concomitantly, the 

 region of first emergence and last resubmergence would suffer most ero- 

 sion. Again, it must not be forgotten that the borders of the emerging 

 continents, as well as occasionally some sections within the continents, 

 still remained submerged at the end of the retreatal movements, and that 

 hence sedimentation there was entirely continuous. Those regions may 

 in many cases be beyond the confines of the present dry-land masses, but 

 in others they are undoubtedly within the present borders of these lands, 

 though erosion during later periods may have removed the record. To 

 use such disconformities as limiting planes for geological systems when 

 unaccompanied by decided changes in faunas seems at present hardly 

 warranted, though it must be conceded that if the exact limits of the re- 

 treatal and readvance movement could be determined, provided such 

 movements were everywhere uniform or with but minor oscillations, and 

 if more or less marked faunal changes could be proved to accompany such 

 changes, a very satisfactory and convenient basis for subdivision of the 

 geological column would be furnished. If such a basis could be adopted, 

 the systems would include the deposits formed from the beginning of the 

 transgressive movement to its culmination, and then to the end of the 

 regressive movement. Each system would thus be separable into two 

 divisions, one representing the transgressive and the other the regressive 

 movement. Since such movements were, however, very probably not all 

 of the same magnitude, our systems would be of unequal value; and while 

 some might be sufficiently great to be accompanied by great faunal changes 

 (more or less induced by these physical changes), others might show no 

 such parallel effects in the organic world. 



In the case of the formations now included in the Ordovicic, we would 

 have three diastrophic systems — the Tremadoc-Arenig, the Llandeilo- 

 Caradoc, and the Ashgillian. The last of these is the smallest in point 



