388 THE CAMBRIAN. IbullSL 



PROBLEMS OF NOMENCLATURE AND CLASSIFICATION. 



The name. — It is still a debatable question with a few geologists, 

 whether we shall follow the present general usage and use the name 

 Cambrian group, with tbe area of Cambrian in Wales as its type and 

 the descriptions of Prof. Adam Sedgwick as the classical description of 

 the group, or, in opposition to this, adopt the name Taconic, proposed 

 by Dr. Ebenezer Emmons, with the area of Berkshire, Rensselaer, and 

 Washington Counties, east of the Hudson River, in New York, as the 

 type, and the papers of 1842 and 1844 of Dr. Emmons as the classical 

 description of the group. 



While recognizing the fact that the name Taconic has certain claims 

 upon the sentiment of Americans, I have stated elsewhere that I think 

 the interests of geology are best served by adopting the term Cambrian, 

 as has been done by the great majority of geologists. 



The limit of the group. — The delimitation of the upper limit of the 

 group by geologists who recognize it as distinct from the superjacent 

 Silurian (Ordovician) has varied to the extent of placing the arenaceous 

 deposits of the shore line, or the strata referred to the Potsdam sand- 

 stone, in the Silurian, or including the superjacent Calciferous forma- 

 tion in the Cambrian. To a large extent this difference of opinion is 

 based upon the evidence of the local physical conditions of sedimenta- 

 tion, and not upon the included faunas. If we adopt as the principle of 

 classification that the delimitation of the great geological groups must 

 rest upon the broad zoological characters of their included faunas and 

 not on local stratigraphic breaks between certain series of rocks, or on 

 local differences of sedimentation, the line of demarkation between the 

 Cambrian and .Silurian (Ordovician) is to be drawn where the marked 

 characters of the Cambrian fauna give way to those of the Ordovician. 

 That this principle is the only sound one upon which to base the delim- 

 itation of a group is proved by the fact that there is no other relatively 

 constant character upon which to rely in geologic classification. That 

 it is often arbitrary is known to all working geologists. 



The line of demarcation along the northern and eastern sides of the 

 Adirondacks is between the upper beds of the Potsdam sandstone and 

 the calcareous layers of the Calciferous for nation. On the south side, 

 in Saratoga County, it is drawn in a series of limestones, where the 

 characteristic Upper Cambrian fauna disappears and the types of the 

 Silurian (Ordovician) fauna appear. In western Vermont, 20 miles 

 from where the division is made between the Potsdam and Calciferous 

 formations, the line is drawn in a series of black argillaceous shales, 

 somewhere between a horizon carrying characteristic Upper Cam- 

 brian fossils and another horizon some distance above, in which 

 characteristic Calciferous fossils are found. In the Southern Appala- 

 chian Province, in East Tennessee, the line of demarcation is not at the 

 base of the limestone series, but at a point in the lower portion of it, 



