200 CHAREES: D. WALCOTT: 
section of the Cordilleran area mentioned on p. 197. This fauna 
was undoubtedly present on the continental shelves to the north and 
south, and may have been distributed around the southern extremity 
of the central land-area to the Hudson and Champlain valley region. 
Future investigation may thus prove that the Holmia asaphoides 
fauna’ of eastern New York is the oldest part of the Olenellus fauna 
upon the eastern side of the continent, and that it may be compared 
with the Holmia rowei fauna of the Cordilleran area. The presence 
in both localities of genera belonging to the Archaeocyathinae indi- 
cates that warm currents were passing through the straits or sounds 
to the east and west of the central continental areas, and that condi- 
tions were favorable for a varied fauna. The arenaceous beds (with 
ripple-marks and trails) of the western Nevada-California area and 
the interformational conglomerates of eastern New York prove the 
presence in both areas of relatively shallow water. 
The Olenellus thompsoni fauna,’ of late Lower Cambrian time, is 
widely distributed about the margins of the continental area. Begin- 
ning at the Straits of Belle Isle on the northeast, it has been found in 
eastern Massachusetts, western Vermont, eastern New York, eastern 
Pennsylvania, and along the Appalachian area as far south as central 
Alabama. In the Cordilleran area it is known to extend from Inyo 
County, California, to the Wasatch Mountains of Utah, and north- 
ward to the line of the Canadian Pacific Railway in British Columbia. 
With the exception of vertebrates, echinoderms, and cephalopods, 
the class-characters of the early Lower Cambrian fauna of Nevada were 
well advanced toward the varied and rich fauna of the lower Ordo- 
vician. 
CONDITIONS DURING MIDDLE AND UPPER CAMBRIAN TIME 
The physical conditions of the late Lower Cambrian time continued 
into early Middle Cambrian time, followed during Middle Cambrian 
time by a gradual submergence through erosion and probable warping 
of the surface of most of the continental area south of the Great Lake 
region. As the marine waters slowly encroached upon this great 
area and upon the shores adjoining the Appalachian and Cordilleran 
t Tenth Annual Report, U. S. Geol. Survey, 1891, p. 570. 
2 [bid., p. 569. 
3 Am. Jour. Sci., Vol. XLIV, 1892, pp. 56, 57. 
