Haworih on the Archcvan Geology of Missoiiri. 363 
cated the priority of the term Taconic, of which the Cambrian 
above mentioned [of primordial fauna] would be the equivalent. 
To us the question seems demonstrated. In such a case, the 
term Cambrian would be employed to replace the Ordovician, 
and the name Silurian would come back by right to group 6. 
If we be not in error, this solution would avoid many difficult- 
ies." 
3. Positio7i and Equivalences of the Taconic system. I for- 
bear the presentation of any extended table of sub-divisions and 
equivalents of the Taconic system. After the presentation of 
Mr. Walcott's results, it is scarcely necessary to do more than 
refer for comparison to the table of M. Marcou in his last pub- 
lication.' I conclude by saying, that in my opinion, the proposal 
of M. Dewalque is entirely feasible and just. The term Or- 
dovician has no need to appear in American geology. It stands, 
in England and America, for the rocks first named Cambrian; 
and Taconic stands for the older strata holding the primordial 
fauna. I favor, therefore, the following arrangement: 
III. Silurian [^Upper Silurian, containing 3d fauna.] 
II. Cambrian [=Ordovician, containing 2d faun.] 
I. Taconic [Containing primordial fauna.] 
A CONTRIBUTION TO THE ARCH^AN GEOLOGY OF 
MISSOURI. 
BY ERASMUS TIAWORTH. 
PART II. 
B. The Porphyries and Porpiiyrytes. 
The porphyries and porphyrytcs constitute at least nineteen- 
twentieths of the massive rocks of Missouri. They are the 
most extensively developed around Pilot Knob, and Iron Moun- 
tain. The different varieties grade into each other by almost 
imperceptible transitions, not only in the character of the rocks, 
but also with reference to the geographical areas over which 
they are exposed. In general the most acid types are near 
1 On the Use of the Term Taconic^ Proc. Soc. Nat. Hist., Boston, xxiii, 
Mar. 2, 1S87, pp. 343-55- 
