WALCOTT] SYNOPSIS. 359 



Owing to the uncertainty as to the Cambrian age of the outcrops of Nova Scotia, 

 Maine, New Hampshire, and north into Canada, the indicated areas npou the map 

 have horizontal black lines to distinguish them from the known Cambrian rocks. 



SYNOPSIS OF THE CAMBRIAN GROUP. 



The coordination of the Cambrian group, in America, with the pri- 

 mary subdivisions of the Paleozoic — Silurian, Devonian, and Carbon- 

 iferous — is largely a development of the past decade. Although strongly 

 advocated by Dr. T. S. Hunt and used by Sir J. W. Dawson in 1872, 

 and the Fortieth Parallel Survey in 1878, Le Conte's Elements of Geol- 

 ogy of 1878 mentions, but does not use the name in classification, and 

 Dana's Manual of 1876 classes the " Primordial or Cambrian " as the 

 lowest formation of the Silurian. 



To-day the group is firmly established by (a) the presence in the 

 Kocky Mountain province of 6,000 feet of limestone with 10,000 feet or 

 more of quartzite beneath ; (b) in the Appalachian Province by over 

 12,000 feet of quartzite, shales, slates, and limestones ; (c) a continental 

 distribution; (d) a characteristic, highly differentiated fauna. 



The various formations referred to the group are mentioned in the 

 following table. In some instances it is not practicable clearly to de- 

 fine by name the exact limitations of the formation through which a 

 certain division of the fauna ranges. In such cases reference will have 

 to be made to the text for further description. 



