ORDOVICIC PERIOD OF AUTHORS 525 



be made in order to bring out clearly the geographic position of the 

 various transgressions and emergences of the Ordovician seas. That this 

 subject is very complex may be seen from the detailed description by 

 Ulrich, given elsewhere in the present volume. 



The fact that there were so many inundations and emergences, with 

 hordes of faunas local and provincial in character, makes it evident that 

 Ordovician time was of far longer duration than is usually supposed. 

 There seem to be here represented about 25 per cent of Paleozoic time, a 

 length in strong contrast with the duration of other periods. 



In constructing these maps a great mass of unpublished data has been 

 utilized, accumulated chiefly by Ulrich in connection with the collections 

 of the U. S. Xational Museum and the IT. S. Geological Survey. During 

 the past four years these maps have been subjected to frequent discussion 

 and emendation, yet are still inadequate, as far as presenting a final and 

 definitely determined geographic distribution of the various faunas is 

 concerned. 



In the Appalachian region are shown topographic forms unknown to 

 the present world, for it has been found necessary to assume long and 

 narrow land barriers, on either side of which are formations both phys- 

 ically and faunally dissimilar. In studying these much constricted bar- 

 riers, it must be borne in mind that the geographic base on which these 

 times are plotted is that of today. That the entire Appalachian and 

 Piedmont region has been foreshortened many miles is well known ; over- 

 thrusts up to 25 miles in length can also be pointed out, and here may 

 be repeated the statement of Claypole that eastern Pennsylvania has 

 been abridged at least 75 miles, yet no one is able to state how great has 

 been the reduction in the total northwesterly thrust and crushing of 

 Appalachia, Acadia, and their Piedmonts. Therefore, if a geographic 

 base could be projected which truly represented Ordovician geography, it 

 would undoubtedly be seen that these narrow ridges not only widened 

 considerably, but took on forms not now suspected. Whatever shape the 

 barriers may have assumed, however, those here illustrated serve merely 

 as means to bring out the fact that synchronous faunas wholly distinct 

 now lie close together. 



In Arkansas and at times also along the southwestern side of Appa- 

 lachia from Alabama into southern Virginia, occur Lower Ordovician 

 faunas which it is thought can not be associated with those of the Pacific 

 realm. They have the Atlantic impress, and are derived from the same 

 faunas as those found in the Saint Lawrence sea and to the east of lake 

 Champlain and the Hudson river. In the main these are graptolite 

 faunas, having many species in common with Great Britain and Sweden, 



