RELATION OF THE CAMBRIAN TO THE ORDOVICIAN. 33 



The Tsi-nan formation is referred to the Ordovician on the evidence 

 of fossils found in its upper member. No fossils were found in the lower 

 portion. 1 



The transition from the Upper Cambrian to the Tsi-nan formation is 

 not marked by an unconformity, but the introduction of argillaceous and 

 dolomitic limestones indicates a change in sedimentation that was brought 

 about by diastrophic action that revived erosion and ultimately led to the 

 great epeirogenic changes that marked the close of the Sinian. The fauna 

 of the Cambrian disappeared, so far as known, everywhere in the western 

 Pacific Province and the faunas of Ozarkiair and Canadian time did not 

 flourish in the Tsi-nan sea, and apparently entered it only at rare intervals. 

 It may be that faunas corresponding to the Lake Champlain and Mississippi 

 Valley Canadian and Ozarkian will be found on the Asiatic continent, but 

 at present we must be content to close the Cambrian with the upper horizon 

 of the Kiu-lung group, and wait for further data on the faunas of the Tsi-nan 

 formation and their relation to the Cambrian and Lower Ordovician faunas 

 of North America and Europe. 



The presence of the genera Syntrophia, Iliioiclla, Cyrtoceras, and 7//<r- 

 nurns in the Ch'au-mi-tien limestone proves that the Upper Cambrian fauna 

 was beginning to assume a post-Cambrian aspect toward the close of the 

 deposition of the Ch'au-mi-tien limestone. It is quite possible that the fauna 

 of the lower portion of the Tsi-nan formation, when found, will have an 

 Upper Cambrian aspect, but it is more probable that it will have the general 

 facies of that of the lower Pogonip of the Nevada Cordilleran sections. 3 



At present the trilobitic fauna of the Upper Cambrian in the Pacific and 

 Cordilleran provinces is readily recognizable at nearly all localities by the 

 presence of such genera of trilobites as Ptychaspis, Illccnurus, and various 

 genera of the Ptychoparidae. Dikelocephalus is restricted in geographic dis- 

 tribution to a few localities in North America. I would place the formations 

 containing a typical Cambrian trilobitic fauna in the Cambrian, and where 

 a formation has a fauna characterized by a new group of forms that evidently 

 belong to a later fauna it should be assigned to a post-Cambrian system even 

 though it may have a few Cambrian genera of trilobites included in it. 



In North America we find that the fauna of the Upper Cambrian in the 

 Cordilleran region is quite distinctly marked by the presence of typical 

 Cambrian genera and the absence of typical post-Cambrian genera. In the 

 central area between the Rocky Mountains and the Appalachians the Upper 

 Cambrian fauna as characterized by the trilobitic genera A gnostus, Ptychaspis, 

 Dikelocephalus, Ptychoparia, and Illcemirus is singularly free from comming- 

 ling of typical post-Cambrian genera except in the case of the Gasconade 

 fauna, where a few trilobitic genera, notably Dikelocephalus, have persisted 

 into Ozarkian time. 4 



1 Blackwelder, 1907, pp. 44-46. 'Walcott, 18846, p. 3. 

 'Ulrich, 1911, p. 627. 'Ulrich, 1911, p. 631. 



