1886. ] Geology and Paleontology. 801 
Upper Cambrian, or Potsdam, the characteristic forms of which 
occur 4500 feet above the Olenellus horizon. 
On the Grand cañon of the Colorado the Tonto beds (1000 
feet) carry a fauna akin to that of the Upper Cambrian of Nevada, 
Texas and the Upper Mississippi valley. Beneath the Tonto lies 
the Grand Cafion series, which Mr. Walcott classes, together with 
the Keweenaw of Wisconsin and the Llano of Texas, as pre- 
Cambrian. 
The Eureka and Highland Range sections in Nevada and the 
Georgia section in Vermont prove the stratigraphic position of 
the Middle Cambrian. In Nevada the Middle Cambrian fauna is 
from 2000 to 4000 feet below the Potsdam, and only three spe- 
cles pass up into it. In the Georgia (Vt.) section one species 
Genera. Species. 
Upper Cambrian. 52 213 
Middle ee a an 43 107 
Mee a 32 76 
127 396 
Reappearances .... 35 3 
Total fauna. 92 393 
ii 3 
Spongie,.... : 3 
Hydrozoa Serek esra $ : 
oaa a z ~ 
UE a a : ; 
Brachiopoda R Sd 3 zi 
Lamel libranchiata,..... |. s : 
Gastropoda ....._ = ri 
€ropoda ....... ve 
Crustacea... s 28 
re wail Sa 31 226 
92 393 
br urteen genera are common to the Lower and Middle Cam- 
we fifteen to the Middle and Upper ; eleven to all three divi- 
Teme genera of the Upper Cambrian, viz, Lingula, Orthis, 
Ptæna, Triplesia, Bellero on, Euomphalus, Holopea, Maclu- 
