wAuxvrr.l PROBLEMS FOR INVESTIGATION. 385 



sections of the eastern ranges of Colorado, Wyoming, and Montana, 

 for the purpose of detecting the presence and character of the fauna as 

 a means of correlating the strata with those of the Black Hills and the 

 upper Mississippi Valley. On the western side, in southeastern Idaho, 

 the Gallatin Kiver section needs special investigation in order to cor- 

 relate it with the sections of Nevada. Reference to what is required in 

 Utah, Nevada, and Colorado has already been made. 



Arizona. — It is desirable tuat larger collections should be made from 

 the lower sandstone of the Cambrian section in the Grand Canon area, 

 and that the geographic distribution of the Cambrian in western Arizona 

 be determined. 



Upper Mississippi Valley. — Through the work of the Wisconsin and 

 Minnesota surveys our knowledge of the Upper Cambrian sandstone is 

 unusually complete. It is desirable that more information should be 

 obtained in relation to the distribution of the fauna in the section and 

 to ascertain whether the Middle Cambrian fauna accompanied the in- 

 vasion of the sea that deposited the lowest beds of the terrane. 



To determine the age of the sandstones on the south shore of Lake 

 Superior it is still necessary to obtain paleontologic evidence to enable 

 the geologist to state positively that the sandstones are the geologic 

 equivalent of the fossiliferous sandstone of the central area of Wiscon- 

 sin. It is also desirable to trace the continuation of the recognized 

 fossiliferous Upper Cambrian sandstone directly west of Green Bay, 

 Wisconsin, with that occurring on the south shore of Lake Superior. 

 The definite settlement of these two problems will remove what has long 

 been the source of contention among geologists. 



Missouri. — It is desirable that the exact line of demarcation between 

 the Upper Cambrian and Silurian (Ordovician) zones should be deter- 

 mined and more positive paleontologic data be obtained of the relative 

 stratigraphic position of the beds with those of other areas that are 

 now referred to the Cambrian. 



Texas.— The information in relation to the Cambrian area of central 

 Texas, in Llano County, is sufficient for most geologic purposes, although 

 more detailed information as to the horizontal distribution of the fauna 

 and the presence or absence of the Middle Cambrian fauna is desirable. 

 The principal question, however, is the investigation of the supposed 

 Cambrian in the vicinity of El Paso and the southwestern portion of 

 the State. 



PROBLEMS AFFECTING OUR KNOWLEDGE OF THE CAMBRIAN GROUP 

 AS A WHOLE OR IN LARGE PARTS. 



A glance at the map delineating the distribution of the Cambrian 



strata as shown by outcrops in North America, PI. i, reveals at once 



the fact that we are dealing almost entirely with a marginal or coast 



series of sediments that are largely covered, over great areas, by later 



Bull. 81 25 



