168 REPORT OF THE AMERICAN COMMITTEE. 
that, " Broadly speaking, all the Archean rocks I have met with 
have a character of their own, and however nearly they may be 
approached by late formations, distinctions can easily be made." 
Prof. Pumpelly thinks there are some. 
Prof. A. Winchell: "Probably not; but I do not speak on 
this subject with adequate knowledge." 
Prof. Dana, Sir W. Dawson, Dr. Hunt, Prof. Irving, 
Prof. Hitchcock, Prof. Emerson, and Capt. Dutton hold 
more or less strongly to the view that there are crystalline rucks 
in the Archean which are not repeated later. 
Prof. Wadsworth is alone in pronouncing distinctly that 
he knows of no crystalline rocks in the Archean which do not 
occur later; Prof. A. Winchell thinks it unlikely that there 
are any; Prof. Le Conte and Dr. Emmons, while recognizing 
a general common character to the crystallines of the Archean, 
are not willinp; to commit themselves to the view that this char- 
acter is never repeated later. 
It would seem, nevertheless, that the bias of sentiment was de- 
cidedly towards the affirmative of the proposition. 
I. Is MINERAL constitution INDICATIVE OF GEOLOGICAL 
AGE? 
Most of the replies to this are so cautious that it is difficult to 
extract from them an unqualified opinion. 
Prof. Dana saj^s: "It is after the demonstration is complete 
that the rock does not occur in any other age or jjeriod.'' [The 
italicized words are made so by the Reporter. Reading this reply 
in the light of Prof. Dana's reply to the last question, it would 
seem to mean that there is at present a limited number of crys- 
talline rocks which have thus far been proved to exist only in 
certain ages and periods, and which are therefore characteristic of 
such ages and periods. Furthermore, the list of them can be 
extended when other rocks are as thoroughly known. — Rep.] 
Sir J. W. Dawson answers: "Locally and with some limita- 
tions, except in the Lower and Middle Laurentian, which, so far 
as my experience goes, are the same everywhere." [Therefore, 
generally as to these. This makes the substance of this reply 
similar to that of Prof. Dana. — Rep.] 
Dr. Hunt says: " The mineral composition of the indigenous 
crystalline rocks is determined by geological age." 
