walcott.J PROBLEMS FOR INVESTIGATION. 389 



where the characteristic Upper Cambrian fauna disappears and the 

 Silurian (Ordovician) fauna begins to predominate. This same prin- 

 ciple of demarcation is followed in the limestone series of central 

 Nevada, while over the interior continental area it is largely drawn at 

 the summit of the sandstone carrying the Upper Cambrian fauna and 

 at the base of the limestone in which the Calcilerous fauna occurs. In 

 some instances, however, the Upper Cambrian fauna extends up into 

 the Calciferous beds, and in such the line of demarcation occurs in the 

 Calciferous zone. 



It is not always possibl e to delineate on the map such close lines of 

 demarcation within a formation, and for the sake of convenience it 

 may be well to map the formation by their lithologic characteristics and 

 describe in the text the line of delimitation between the groups. When, 

 however, the liue between two groups occurs in a limestone, as in the 

 Pogonip limestone of the Eureka district, Nevada, and there is suffi- 

 cient paleontologic evidence to demonstrate it, the line should be drawn 

 on the map at the point indicated by the faunas. 



The problems in connection with the delimitation of the summit of 

 the Cambrian group remain to be studied in nearly all portions of the 

 various provinces. In some they are fairly well settled, but there still 

 remains much to be done in the way of collecting the faunas and study- 

 ing their verfical distribution in the strata. 



Is the basal line of the Cambrian group at the lowest limit at which 

 the Olenellus fauna is found "l Where this lowest limit occurs at the 

 base of a conformable series resting unconformably upon pre-Cambrian 

 rocks there is no difficulty in answering the question. "But where it 

 occurs in the midst of a conformable series and there remain thousands 

 of feet of sediments beneath the Olenellus zone, as in Nevada and Utah, 

 it still remains a problem for consideration. On the general proposi- 

 tion that I would not refer to the Cambrian any rocks beneath the 

 Olenellus zone, the subjacent strata of the western Rocky Mountain 

 area were referred to the Algonkian in the paper reviewing the Lower 

 Cambrian or Olenellus fauna published in the tenth annual report of 

 the U. S. Geological Survey. I think, however, it is a question for dis- 

 cussion and research, as the Olenellus fauna has a great vertical and 

 time range in the Northern Appalachian Province, while in the South- 

 ern Rocky Mountain Province it is limited to the very narrow band of 

 strata at the summit of the siliceous series of slates and quartzites. 



That the question of how the Cambrian shall be divided in America 

 still requires investigation, with special relation to the classification 

 to be applied in the various provinces, is well known. The three pri- 

 mary divisions of the Cambrian (Lower, Middle, and Upper) are readily 

 recognized in Newfoundland, and that one or the other of these divi- 

 sions can be distinctly recognized in the other provinces has been 

 proved. 



In the Interior Continental Province only the Upper Cambrian, with 



