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." 
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 Ozarkian’ 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, Huenella, Cyrtoceras, and Ille- 
nurus in the Ch’au-mi-tién 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-tién 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.’ 
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, Illenurus, and various 
genera of the Ptychoparide. Dzkelocephalus 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 Illenurus 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. 

1 Blackwelder, 1907, pp. 44-46. *Walcott, 1884), p. 3. 
2 Ulrich, 1911, p. 627. ‘Ulrich, 1911, p. 631. 
