46 THE CAMBRIAN AND ORDOVICIAN DEPOSITS OF MARYLAND 



all of the maps covering the formations here under discussion. These 

 are inserted in their appropriate place in the text. 



It must be remembered that in these maps the shore lines are more or 

 less hypothetical and do not show the bays and other features of present- 

 day strands. It must also be remembered that the present-day base maps 

 do not accurately represent the continents of past times, because the latter, 

 especially in the mountainous areas, have suffered great compression. 



Although it is evident that the portion of Maryland covered by the 

 Cambro-Ordovician rocks is too small a part of the North American 

 continent to reveal much of the paleogeography, still it may be noted 

 from the accompanying maps that western Maryland has been much 

 concerned in the continental oscillation and other earth movements which 

 occasioned the repeated invasions and withdrawals of the sea. In order 

 to plot these sea invasions not only must the distribution of the marine 

 sediments of the time be determined and the ancient shore lines thus 

 approximated, but also the particular oceanic basins from which the 

 several fossil faunas have migrated must be ascertained. The waters 

 which have repeatedly flooded the continent have come from the Arctic, 

 Atlantic and Pacific oceans and the Gulf of Mexico, and each brought with 

 it such samples of its own particular life as were available and suited to 

 existence in shallow epicontinental seas. 



Comparative studies of the fossil faunas have shown them to have had 

 a considerable sameness in composition when derived from the same 

 oceanic source, and to have had great unlikeness to contemporaneous 

 faunas that originated in other oceanic basins. This appears to indicate 

 not only that the life in the several oceanic basins evolved more or less 

 independently, but also that each maintained in recognizable measure 

 its individual characteristics. These distinctive facies were perhaps never 

 less and may often have been greater than now. At any rate Ordovician 

 faunas of Arctic origin are at least as distinct from approximately con- 

 temporaneous ones of Gulf origin as the life of the Arctic Ocean to-day is 

 different from that of the Gulf of Mexico. Appreciating these distinc- 

 tions paleontologists are succeeding very well in discriminating the faunas 



