234 CONFERENCE ON PALEOZOIC PALEOGEOGRAPHY 



ably outside the reach of the waves and in moderate depths of the sea), 

 and both were dependent on oceanic currents for transportation, it i& 

 thought impossible that they could have entered culs-de-sac such as the 

 Baltic Sea in any considerable quantities, and their abundant and con- 

 tinued presence is taken as indicating the deposition of the beds in either 

 the ocean or a sea with free egress and ingress. 



Scattered occurrences of graptolites may occur in epicontinental seas 

 by the slow action of an entering surface current in a tropical hypersaline 

 sea, such as the Eed Sea of today, or through an entering bottom current 

 in a brackish sea, as the Baltic Sea now is. 



The black carbonaceous graptolite shales do not indicate conditions of 

 a nearly inclosed basin, such as is now exampled by the Black Sea, for in 

 the latter life exists only near the surface, and the Axonophora, at least, 

 quite surely lived in the more quiet depths, nor would in such a basin be 

 found the great mass of floating seaweed to support the Axonolipa. Many 

 different graptolite zones occur, as a rule, in a small thickness of rock, 

 but sometimes they are also embedded in coarser sediments. The most 

 essential requisite for the formation of the black fine grained graptolite 

 shales is, therefore, not the depth, but the tranquillity of the water. The 

 graptolite shales, therefore, indicate a zone between the agitated water, 

 where coarser sediments are deposited, and the dead or currentless water, 

 of the deeper sea. Their longitudinal distribution, then, also indicates the 

 direction of a coastline, which has to be sought on the farther side of a 

 parallel band of coarser littoral sediments, and two such flanking littoral 

 bands may be looked for in narrow channels like the Levis Channel. 



The last corollary from the world-wide oceanic distribution of the 

 index graptolites is that their zones are not homotaxial, but virtually 

 synchronous, these graptolites thus being index fossils for absolute time 

 correlation. This conclusion postulates that the new graptolite species 

 either developed in all oceans pari passu, or that if new forms originated 

 in one oceanic basin they so rapidly spread into the others that deposition 

 of rock did not take place sufficiently quick to record this migration in 

 the rocks. The former hypothesis of the uniform development of the 

 index graptolites in all oceans would presuppose the absence of any 

 physical differences in the oceans, and it fails to account for the provin- 

 cial forms; the latter hypothesis, of the immediate diffusion of new 

 forms, is apparently not quite supported by all the facts, for some of the 

 common graptolites do actually appear later in one basin than in another. 

 It is thus claimed, to cite one instance, by T. S. Hall that in Australia 

 Loganograptus logani appears much later in the graptolite horizons than 



