296 E. O. ULRICH REVISION OF THE PALEOZOIC SYSTEMS 



the south by El Paso, Texas, and on the north by the Arctic regions. 

 Everywhere it seems confined to a bed less than 50 feet thick. 



Another striking instance is the Lowville (Birdseye) limestone, which 

 has been recognized by identity of fauna and extraordinary persistence of 

 lithologic characters from Canada to Alabama, and westward to north 

 Arkansas and east Missouri. Every feature connected with the forma- 

 tion as developed in middle Tennessee and central Kentucky — dominant 

 fossils, succession of faunules, lithology, a sharp but not conspicuous 

 break beneath separating it from upper Stones River limestone and an- 

 other more conspicuous but less important hiatus above — can be dupli- 

 cated in the typical outcrops of the formation in New York. If this 

 parallelism does not establish essential contemporaneity, then we may as 

 well cease trying to correlate. 



A third instance, differing decidedly from the preceding in that it 

 involves transgression westward across barriers, is afforded by the Mar- 

 tinsburg and Utica shales. The Martinsburg shale occupies the interval 

 between the Massanutten sandstone above and a limestone beneath. On 

 faunal and diastrophic evidence the last bed of this underlying lime- 

 stone is proved to be late Black River or earliest Trenton in age. The 

 lower part of the northern Appalachian equivalent of this shale has been 

 generally referred to in New York and elsewhere as Utica, but it is in 

 fact much older. The true Utica is found well up in the eastern shale 

 series. Eastward from Little Falls, New York, more and more of Tren- 

 ton time is represented by shale, but to the west of that town we find 

 limestone instead of shale. The typical Utica finally rests on a full 

 development of Trenton limestone, and thus is proved to represent a later 

 stage in the westward transgression of the almost continuous eastern 

 shale sedimentation. 



The case of TriartJirus hecki. — The trilobite Triarthrus hechi has 

 nearly always been regarded as the most characteristic fossil of the Utica. 

 It is, as indeed is the whole genus, a migrant from the Atlantic ; and it 

 seems to have been an excellent traveler, since we find it generally with 

 the advance guard. The first known occurrence of this species in America 

 is in the basal 10 feet of the Martinsburg. When the eastern sea again 

 transgressed westward during the true Utica this ambitious rover came 

 in on the first wave. That Triarthrus becki maintained this character- 

 istic is shown by its abundance at Cincinnati, Ohio, in the 1 to 26 feet 

 of Fulton shale which represents the extreme southwestern limit of the 

 true Utica invasion. 



This trilobite reappears in the Cincinnati section in the Southgate 

 member of the Eden shale approximately 150 feet above its occurrence in 



