THE GEOLOGICAL SOCIETY OF WASHINGTON
could have been synchronous, since there is no evidence of intermingling of species characterizing the respective formations in the median areas of interfingering overlaps. It is only the pelagic and semi-pelagic types that can be depended upon for exact correlation between widely separated areas, and prove the existence of unobstructed current-highways. Referring to such organisms, the distribution of the Eopaleozoic graptolites offers very strong arguments against the hypothesis of
... hesis of transcontinental currents in the interior basins. The most important of the graptolite faunas are confined to current-swept submarginal channels. Had these currents continued across the continental border, as assumed by Willis, the graptolites must have been carried by them into and through the interior seas, a condition wholly negatived by the evidence in hand. Perhaps the strongest argumnent against the efficiency of currents in preventing deposition in the interior continental seas is found in stratigraphic overlaps. In any period of sea-advance, beds are deposited by overlap toward the ' positive" or relatively elevated areas. At the same time, the submergence increasingly favors the formation of currents. If currents were present and competent to cause scour, phenomena quite opposed to those observed would obtain. The lower beds, which are absent. should be present, and the later beds, formed in a sea supposedly favorable to the extensive development of strong currents, should not be deposited, or should show the effects of current action by diminished thickness.