THE ARCHEAN. 169 
Prof. Le Conte thinks that it is a pynori probable, but he 
does not believe that it is a safe guide in the present condition of 
our knowledge. 
Prof. Irving thinks, not further than he has indicated under 
Question G. — [i.e., that the post-Paleozoic crystallines differ from 
the great Archean gneiss, which is simulated, but not repro- 
duced — and then only doubtfully.] 
Dr. Emmons thinks, not of absolute age, though it may serve 
as an indication of relative age. 
Mr. Walcott answers : " Not in my experience." 
Prof. Geo. H. Williams says : " I do not believe, except 
in a broad way, that petrographical character in the crystalline 
rocks is a safe guide to their geological age." 
Prop. Hitchcock says : " I am disposed to believe that we 
shall be able to use mineral constitution as defining the different 
parts of the Archean. The facts are yet to be made out, but 
Sterry Hunt has made, lately, many suggestions that cannot be 
lightly set aside. We cannot, however, pick out a particular 
mineral like Andalusite and make it characteristic of a horizon, 
as Paradoxides is of the older Cambrian." 
Prof. N. H. Winchell says : " I think I can identify any 
Minnesota Archean terrane by a hand-sample that may be pre- 
sented to me, and I think there is much more reliance to be 
placed on mineral constancy and association with other minerals, 
in the matter of determining Archean stratigraphy, than has been 
allowed (as by Prof. Dana) in recent years." 
Prof. Wadsworth answers decidedly: "No." 
Prof. Emerson answers, " No," in general. Only within 
limited areas. 
Capt. Dutton says : " As far as my experience goes, Archean 
rocks have characters which seem to be distinctive, and which 
raise a presumption in favor of the view that this distinction is 
to be found partly in mineralogic characters. But I do not 
think we have sufficiently investigated the subject to enable any 
one to say with certainty that this is so, and at the same time 
point out precisely wherein this distinction consists. As regards 
all later strata than Archean, I do not believe that any valid 
mineralogic distinctions can be found. I have seen Middle 
Cambrian beds which the very elect could not distinguish litho- 
logically from Green Piver Tertiaries." 
