FOSSILS OF THE DOLOMITES 231 



faunas are altogether too meager to permit their being used as a basis 

 for any conclusion in the matter. In the case of certain Devonian dolo- 

 mitic formations — as, for instance, the Middle Devonian beds at Mil- 

 waukee, Wisconsin, and the dolomitic beds which are present in the 

 Devonian section of Iowa — we have faunas constituted of species which, 

 almost without exception, occur elsewhere in calcareous sediments. The 

 same is true of certain Ordovician formations other than the Galena. 



In conclusion it may be stated that from the evidence of the fossils 

 alone there seems to be no reason for assuming that our widespread dolo- 

 mitic formations of Paleozoic age have been deposited under conditions 

 which are notably different, as regards salinity, temperature, or depth, 

 from those under which non-magnesian formations, either argillaceous 

 or calcareous, have been laid down. Chemical geologists are almost 

 unanimously agreed that in general the dolomitization of limestone is a 

 secondary process, and the paleontological evidence, so far as it is avail- 

 able, seems to substantiate that view. Formations now dolomite were 

 in all probability originally deposited as limestones, and have been 

 altered to dolomites since their original deposition, while other beds 

 entirely similar in original condition have not been modified, but persist 

 to the present time as true limestones. 



STRATTORAPHIG SIONIFICANGE OF THE WIDE DISTRIBUTION OF 

 GRAPT0LITE8 ^ 



BY RUDOLF RUEDEMANN 



I have been asked by your President to speak on the stratigraphic sig- 

 nificance of the wide distribution of graptolites. 



This title assumes the wide distribution of the graptolites as an estab- 

 lished fact. It is such, but with a certain qualification that will be 

 considered later. 



The closer investigation of the graptolites in Europe, America, and in 

 Australia has brought out the fact of the presence in all three continents 

 of the common or guide graptolites, of the Ordovicic, at least, and of the 

 general agreement of the sequence of the zones. The distribution of an 

 important fraction (roughly, at least one-third) is world-wide. 



The structure of the graptolites has shown that the earlier forms with-* 

 out axes (Axonolipa) were for the greatest part pseudoplanktonic — that 

 is, they drifted about fastened to seaweed — while the later Axonophora 

 were planktonic or floated by means of apparatus of their own. 



Manuscript received by the Secretary of the Society May 23, 1911. 



