070 E. O. ULRICH REVISION OF THE PALEOZOIC SYSTEMS 



layer is sometimes silicified, the residual blocks being exceedingi}' rough 

 and cavernous or spongy. The fossils have not been all worked out, 

 but most of those so far studied are of exceeding interest because of their 

 bearing on correlations. Thus among them are Madurea cf. matutina, 

 Liospira canadensis, Hormotoma artemesia (very large form), Camero- 

 ceras cf . hrainerdi, Protocycloceras lamarchi, Gyrtoceras confertissimum ?, 

 Tarphyceras cf. seelyi and clarkei, Bolhocephaltcs seelyif, Bathyurus cf. 

 conicus and nero, Dalmanella? electra, and Syntrophia cf. lateralis 

 and palmata. 



The second conspicuous layer or zone, when present, occurs from 35 to 

 40 feet beneath the upper. It also is often a gray pitted dolomite, but 

 frequently it is arenaceous and sometimes the corresponding horizon is 

 made up of irregularly interbedded sandstone, quartzite, conglomerate, 

 and chert, aggregating as much as 6 to 10 feet in thickness. No fossils 

 have been observed in this bed. Neither have any been seen in the 

 argillaceous dolomite above, beneath, and between these two layers. 



The fossils mentioned in the foregoing paragraphs as occurring in 

 the various zones of the Yellville, with the exception of the highest — 

 sponge and Bryozoa zone in the Black Eock section — and the lowest or 

 Phyllograptus bed, are so clearly indicative of the Division D or Fort 

 Cassin fauna in the Lake Champlain Beekmantown that the general 

 contemporaneity of these Yellville and middle Beekmantown zones 

 seems undeniable. As to the sponge and bryozoan bed at Black Eock, 

 this may very well represent a time corresponding to some part of 

 Division E of the Champlain section. 



Age of the Phyllograptus and Tetragraptus faunas.^The discovery of 

 diagnostic graptolites of the Phyllograptus illicifolius zone in the basal 

 member of the Yellville formation proves it older than the Cassin fauna. 

 Beyond this, however, and aside from the fact that it is positively younger 

 than the last of the Ozarkian, the age of this almost universally dis- 

 tributed graptolite zone is yet somewhat indefinite. According to other 

 lines of evidence, which will be discussed presently, the Levis graptolite 

 zones, beginning with the Dictyonema flabelliforme zone and including 

 at least the Phyllograptus illicifolius zone, correspond to the whole or 

 parts of the interval between the middle of Division D and the top of 

 Division B of Brainerd and Seely's Champlain Valley section. That this 

 view extends the base of the Levis as far down the column as the lowest 

 Canadian in the Bellefonte, Pennsylvania, section seems not unlikely; 

 but the point can not be decided positively with the evidence now avail- 

 able. According to this evidence we may go only so far as to say that 



