THE ARCHEAN. 165 
them at a distance from each otlier, or to adduce evidence of them 
by lithological resemblances.* 
lu point of fact, if it be once established that there are no 
characteristics of the oldest known crystalline rocks by which 
they may be distinguished from eruptive masses of any subse- 
quent period ; that the underlying granite floor which was the 
first to be solidified, is liable to be softened and pushed upwards 
through flaws in the earth's crust, and when newly established 
becomes indistinguishable from the original mass : the crystalline 
rocks will excite an interest very subordinate to the sedimentary, 
and can give but litle aid to the solution of the questions of the 
later evolution of the planet. 
Prof. J. D. Dana answ^ers: "There are many crystalline 
rocks in and after the Paleozoic which are indistinguishable 
from that of the Archean." 
Sir J. W. Dawson says : " Yes, there are such, but they are 
exceptional, whereas in the Archean they are typical." 
Major J. W. Powell is of opinion that the question is not 
to be settled by a convention of geologists, but by investigation. 
Dr. T. Sterry Hunt denies that there are any indigenous 
silicated rocks in and after the Paleozoic which are indistinguish- 
able from the Archean. 
Prof. Jos. L. Le Conte says: "I am not able to distinguish 
some crystallines of later age from some of Archean age, but I 
do not say that others cannot." 
Prof. Roland D. Irving says : " I suppose that the only 
answer I can give to this is, that I do not know and I do not 
believe that any one else knows. Having said this, I will give 
my notions. 
" Eruptives, of course, should be set aside. They may be simi- 
lar in the Archean and Post-Archean formations. Outside of un- 
mistakable eruptives my own experience — and my own experience 
leads me wholly that way — is that while there are rocks which 
have been altered and have acquired a semi-crystalline character, 
e.g., some mica-schists and quartzites, among the Post-Archean 
formations, counting now the Huronian as Post-Archean — there 
are some true Archean rocks, chiefly the true banded gneiss, 
which do not occur above the Archean. But in saying this I 
* These authors themselves do it on p. 522 of their Azoic System, where they 
alhide to the lithological character of certain rocks as belonging to the Azoic. 
