362 THE CAMBRIAN. [bull. 81. 



BASE OF CAMBRIAN. 



The statements made in the preceding paragraphs foreshadow the 

 conclusion that over the Interior Continental area the basal beds of the 

 upper division of the group rest unconformably upon pre-Cambrian 

 rocks. This is true also of the lower division of the Atlantic Coast 

 Province, but in the Lake Champlain Valley and the Southern Appala- 

 chian Province there is no positive assurauce that the conformable se- 

 ries of strata beneath the Olenellus zone do not pass down into some 

 ■pre-Cambrian group. In Nevada, Utah, Montana, and British Colum- 

 bia the same conditions exist to a more marked degree, and I have, in 

 the absence of paleontologic proof, referred the conformable pre-Olenel- 

 lus strata to an Algonkian group. That a considerable portion if not all 

 of the series will be ultimately referred to the Cambrian is probable 

 from the fact that the Olenellus fauna has been traced downward 

 through 1,000 feet of limestone in the Champlain Valley of Vermont and 

 3,000 feet downward in the Bow River series of British Columbia. The 

 great vertical distribution of the fauna in the limestone leads to the ex- 

 pectation that it will be found to range through a considerable portion 

 of the conformable strata beneath the narrow Olenellus zone of the 

 Southern Appalachian and southern portion of the Rocky Mountain 

 Province. On the map of sections the lower strata are provisionally 

 and doubtfully referred to the Cambrian. This is in opposition to the 

 view advanced in the Tenth Annual Report of the U. S. Geological 

 Survey. 



SUMMIT OF CAMBRIAN. 



The determination of the top of the Cambrian is beset with more dif- 

 ficulties than its delimitation at the base. At the base there are marked 

 unconformities, and in their absence the presence of a distinctly marked 

 fauna beneath which no other fauna has yet been found. At the sum- 

 mit, however, we meet with a transition both in sedimentation and faun 

 to the characteristic type of the Silurian (Ordovician). One of the best 

 illustrations of this is in the Eureka district of Central Nevada, where, 

 in the Pogonip limestone, the passage from the Cambrian to the Silurian 

 (Ordovician) is so gradual that it is only by the predominance of the 

 fauna of one or the other of the two great groups that a line of demarka- 

 tion can be drawn. The transition is such as might be expected where 

 there was no marked physical disturbance to influence the change of 

 faunas resulting from the natural dying out and development of species 

 or the influx of new species from other areas. The sedimentation was 

 very uniform, and no essential change of character occurs between the 

 limestones of the Upper Cambrian and those of the Silurian (Ordovician). 

 On the other hand, the delimitation between the two groups on the 

 northern side of the Adirondacks is clearly and definitely marked by the 

 occurrence of a light-colored, calciferous sandrock, with its distinctive 

 fauna, resting upon a compact, hard quartzite, characterized by the 



