The Taconic or Loirer Cambrian. — Wincliell. 297 
their stratigraphic place is determinable from the attendant 
physical structure. 
It is hardl}' necessary to enumerate categoricallj" the kinds of 
igneous rock which are found in the Taconie. Broadly speak- 
ing they are as a group such as are not found in any other 
geological horizon. They are illustrated by the eruptives at 
Cortlandt, in the valley of the Hudson, in the so-called upper 
Laurentian of the Adirondacks, the upper Laurentian of the 
region of Ottawa and in other places in Canada, and typically 
by the great Keweenawan series of the Lake Superior region.* 
The}^ are also as fairly represented by the eruptives of the 
copper range which extends northward through the "eastern 
townships" of Canada from the Vermont state boundary to- 
ward Quebec, and by the copper and associated felsytes of the 
Sc)uth Mountain region in southern Pennsylvania. That these 
can petrographically be included in the same class or classes, 
distinct from eruptives of earlier or later date, there is abun- 
dant reason to affirm. It would, however, be sufficient for our 
purpose here to simply call attention to the non-existence of 
such rocks in any earlier, or Archean, terrane, since it is only 
in the Archean that they can be found a place if excluded 
from the Taconie. 
It may be well at this place to mention some of the data on 
which this generalization is based. If it be admitted that 
as a petrographic group of igneous rocks a similar assemblage 
may be found in each of the places mentioned, it will only be 
necessary to adduce the evidence of their having essentially 
the same age. 
Beginning with the eruptives of the Cortlandt series, which 
have been described by Dr. Geo. A. Williams,f there is good 
reason to consider them as post-Taconic. This rests on the 
authority of Prof, J. D. Dana, who, at intervals, for several 
years, laboriously traced the Taconie rocks from southern 
Vermont to Cortlandt and to New York city. At Cortlandt he 
*Van Hise says: "Quartz ])()rphyry and certain pluisos of the basic 
eruptives have been found nowhere but in the Keweenaw series." This 
is rather broad, and is inter[)reted tomean nowiiere in llie LakeSiiperior 
region. Witii tliis limitation even it mi.izht not be admitted by some wlio 
have reported feisyte and (piiirtz jjorpiiyries in tiie Keewatin and in tiie 
Animikie. See Men, xix, U. S, (Jeol, Sur., \^. 4()2. 
f Am. Jour.Sci., NoritesoftlieCortlandl series, vol. xxxiii, pp. i:J8, Ii)l, 
24::}, 1887. 
