160 REPORT OF THE AMERICAN COMMITTEE. 
Prof. J. D. Dana says: " Petrographic maps are valuable. 
Petrogra})liic divisions are all conveyed by such maps as far as 
they are important." 
Dr. T. Sterry Hunt thinks that the distinctions ought to be 
at once petrographical and chronological. 
Prof. N. H. Winchell objects that petrographers are not 
yet sufliciently agreed on the meaning of their terms. 
Prof. Wadsworth thinks there is too little knowledge of the 
true petrography of the Archean. 
Dr. Emmons, Prof. Blake, Capt. C. E, Dutton and Prof. 
A. Winchell disapprove of this plan. 
Prof. Pumpelly approves of it only for local distinction. 
Dr. G. M. Dawson thus answers the question : " No, except 
as a local expedient, guardedly applied, in the absence of strati- 
graphical evidences. The Laurentian may, however, prove to be 
an exception to all other formations in this respect." 
The weight of the opinions seems to incline towards using the 
method locally without attempting to establish definite horizons 
over remote areas by lithological distinctions. 
E. Should the eruptives occurring in the Archean 
rocks be classified with the latter or sepa- 
rately? 
The prevailing opinion that many of the Archean rocks, which 
have been heretofore looked upon as sedimentary, are altered 
igneous rocks, and that it is impossible at the present time to dis- 
tinguish the sedimentary from the pseudo-sedimentary, suggested 
the above question which was brought up incidentally in debate 
at Berlin. 
Prof. J. D. Dana thinks that eruptives of each age should 
be colored with some distinguishing shade of the color of that 
age. 
Sir J. W. Dawson in answer to this question writes " some 
bedded aqueo-igneous strata." 
Dr. T. Sterry Hunt says the various eruptive rocks, whether 
plutonic or pseudo-plutonic, should be omitted in classifying the 
Archean. 
Dr. Selwyn thinks that it depends upon whether the age of 
said eruptives can be, or cannot be, determined, and which are 
eruptives, which irruptives, and which simply metamorphic. 
