ART. 21 ORDOVICIAN TRILOBJTES- — ULRICH 77 



from a few feet to more than 100 feet in thickness and is distin- 

 guished from the others by a complete change in the character and 

 in most cases also in the derivation of its famia. The first (Joins) 

 and second (Oil Creek) derived their faunas from the west, whereas 

 the third (McLish) contains species of the Appalachian Lenoir 

 fauna, which therefore are regarded as indicating an invasion from 

 the east at this time. The fourth (Falls) contains species found 

 elsewhere in America only in Nevada and western Texas, which 

 is interpreted as showing that the sources of the invasion was 

 again in the Pacific. The fauna of the fifth formation (Tulip 

 Creek) compares closely only with Stones River faunas of Tennessee, 

 and is therefore held to be an Oklahoma recurrence of that southern 

 fauna during Blount time that did not reach central Tennessee. The 

 fauna of the sixth (Criner) formation again differs radically from 

 that of the next underlying formation. Genetically comparable fos- 

 sils occur only to the east in Blount and Chambersburg formations. 

 Finally, the faunules of the succeeding Bromide formation are essen- 

 tially the same as those found in the Black River and early Trenton 

 formations in Iowa and Minnesota, whose northern origin has long 

 been recognized. 



The position of the succeeding Viola limestone in the time scale 

 can not as yet be fixed with precision. We know, however, that it 

 follows the Trenton, so that it must fall somewhere in the Cincimia- 

 tian or into the hiatus that everywhere separates that series from the 

 Richmond. Its graptolites compare rather well with the Upper Hart- 

 fell of Britain, and its trilobites, among which species of Crypto- 

 lithus predominate, agree better with British Caradoc forms than 

 with any other trilobite fauna known. The Tyner and Sylvan are 

 early Silurian and clearly correspond to parts of the Maquoketa 

 of Iowa. Above these come thin limestones of Upper Medinan and 

 Clinton ages that are 'better developed in eastern Missouri and 

 northern Arkansas. 



Mississippi Valley. — At the base of this column the Ozarkian and 

 Canadian depositional record between the top of the Upper Cam- 

 brian and the base of the Buffalo River series in Missouri is broadly 

 indicated on the left side and the inferior record of the same sys- 

 tems in Wisconsin on the right side. Throughout the valley north 

 of Tennessee limestone of Black River age rests on the Buffalo 

 River series. Evidently, about 10,000 feet of deposits — more than 

 half of this thickness consisting of limestone — that occur in east 

 Tennessee and other parts of the Appalachian Valley are wanting 

 in States bordering the Mississippi. Generalized, but in most cases 

 very detailed correlations of the formations of the Mohawkian series 

 in this colimm with those in Oklahoma, Kentucky, Tennessee, or 



