J. EH. Marr—Origin of Archean Rocks. 263 
TJJ.—Oricin ofr tHE Arcuman Rocks. 
By Joun E. Marr, M.A., F.G.S.; 
of St. John’s College, Cambridge. 
T is somewhat remarkable that, just as physicists are assuring 
geologists that the latter will have to quicken their forces, so 
as to bring the geological history of the globe within the definite 
limits assigned by the physicists themselves, the geologists reply by 
adding several thousand feet more rock in various parts of the world 
to the already thick column of sediment, and many would assure 
us that these ‘Archean’ rocks were produced in a manner similar 
to the fossiliferous rocks, from which they differ in such important 
particulars. | 
Various attempts have been made to account for the mode of 
formation of these Archean rocks under conditions different from 
those prevalent in later times, but little notice has been taken of 
these theories. The two extreme views are perhaps those of Mr. 
Mallet and Dr. Sterry Hunt; but although these authors attempt to 
explain the phenomena of regional metamorphism, they give us no 
clue whatever to many of the other difficulties connected with the 
Archean rocks. Some of these difficulties may be here enumerated. 
1. The amount of time required to accumulate the great thickness 
of Archean rocks, to metamorphose them, and to denude so great a 
quantity of material as to expose the metamorphosed rock at the 
surface would be very great, if these rocks were originally deposited 
as ordinary aqueous sediments. True, according to Mr. Wallace’s 
calculations (Island Life, p. 203), the whole thickness of sedimentary 
rocks could be formed in a time much shorter than that which 
physicists are disposed to allow us. But even admitting the accu- 
racy of Mr. Wallace’s data for estimating the comparative rates of 
removal of a thickness of material from our continents, and its settle- 
ment upon areas receiving sedimentary deposits at the present day, 
there are many possibilities to be taken into account which might 
seriously alter his results. If denudation acted more rapidly in 
past times, deposition on the other hand would probably take place 
over areas more remote from the land than those to which it is 
confined at the present time. Again, if we suppose that continents 
have gradually grown in size, a far smaller amount of land-surface 
would have been exposed to denudation in past times than at present. 
2. The evidence in favour of the permanence of our ocean basins 
is now very strong, gathered as itis from considerations stratigraphi- 
cal, paleontological, physical, and founded upon the geographical 
distribution of life. Mr. Wallace may well say, whilst commenting 
upon this theory, that Mr. Charles Darwin’s argument, framed on 
the absence of Paleozoic and Secondary rocks upon oceanic islands,’ 
“coming in support of the long series of facts of an altogether dis- 
tinct nature, going to show the permanence of continents, the cumu- 
lative effect of the whole must, I think, be admitted to be irresistible ” 
(Island Life, p. 98). ee . 
1 See ante, Mr. Crosby’s article, p. 251. 
