walcott.] SYNOPSIS. 363 



Dikelocephalus fauna of the Upper Cambrian. Various grades of tran- 

 sition occur between these two extremes, but frequently it is necessary 

 to draw an arbitrary line and define the upper limit of the Cambrian, as 

 terminating where, in any continuous section, the Upper Cambrian fauna 

 gives way, and is superseded by the types of the Silurian (Ordovician) 

 fauna. In Nevada this line would be drawn near the base of the Pogonip 

 limestone; in Tennessee, near the base of the Knox dolomite; in north- 

 ern New York, north of the Adirondacks, between the Potsdam sandstone 

 and the Calciferous sandroik ; on the south side of the Adirondacks, in 

 the limestone between the Potsdam and Calciferous faunas; in the Up- 

 per Mississippi Valley, between the upper beds of the St. Croix sand- 

 stone and the Lower Magnesian limestone, or in some instances in the 

 lower portion of the Lower Magnesian limestone ; in the series of shales 

 opposite Quebec, at Point Levis, beueath the graptolitic-bearing shales 

 and the conglomerate carrying the Dikelocepbalus fauna of the Upper 

 Cambrian; and in western Newfoundland, in the limestones between 

 the Upper Cambrian fauna and the base of the Ordovician fauna. 



The paleontologic break between the Cambrian and Silurian groups 

 is practically complete where the stratigraphic break is marked. About 

 the Adirondacks the only species I know of that cross the break are 

 the Lingulepis acuminata and a species of Ophileta, at Chateaugay 

 Chasm. In Wisconsin some of the species of the Upper Cambrian fauna 

 apparently rise into the lower portion of the Lower Magnesian lime- 

 stone, and are there associated with forms which are considered to be 

 more typical of the Silurian (Ordovician) fauna. In Nevada, where the 

 sedimentation is of the same character in the Upper Cambrian and the 

 lower portion of the Silurian (Ordovician), there is a slight blending of 

 the faunas, as shown by the occurrence of the genera Asaphus, Dikelo- 

 cepbalus, and Ptychoparia upon the same surface of rock. With our 

 present knowledge, however, the occurrence of the same species in 

 strata that are referred to the Cambrian and the Lower Silurian is so 

 rare that they are the exceptions to the general rule. 



It is not, however, by the specific break in the fauna that the two 

 groups are separated ; it is the general facies of the fauna referred to 

 the Cambrian and that referred to the Silurian (Ordovician). The dis- 

 tinctness of the Upper Cambrian fauna from the Lower Silurian fauna 

 100 feet beneath the line of contact of the two groups is such that I 

 think there will be no confusion between the two faunas in America. 

 In England the range of the genus Agnostus has complicated the sep- 

 aratenebs of the two faunas, but this is not sufficient to combine them 

 as a whole. 



SEDIMENTATION OF THE CAMBRIAN GROUP. 



The physical and lithologic characters of the strata lead to the con- 

 clusion that the sediments were accumulated in relatively shallow seas 

 in the immediate vicinity of the shores of land areas that were being 

 slowly depressed in relation to the surrounding waters. There are 



