od. EB. Marr—Origin of Archean Rocks. 265 
occupied by more contracted seas in the early Tertiary times, and by 
gulfs ramifying through the centre of the continent in still later times. 
The unconformities separating different systems show that the 
growth was not continuous, the continent being sometimes larger at 
an earlier than at a later period, but in no case are these uncon- 
formities spread all over the continent with great discordance of dip 
and strike, except at the end of Archean times. This is remarked 
by Mr. Gilbert in the place alluded to before. The break at the top 
of the Archean rocks is not only shown by great discordance of the 
strata above and below, but is also marked by the immense amount 
of denudation which took place at the end of Archean times: this 
will be considered shortly. Such being the case, if the theory of 
growth of continents be true for later times, it will not hold good for 
Archean times, if those rocks are of marine origin. 
It is worthy of notice that the two theories just considered—that 
of the permanence of the site of ocean basins, and that of the growth 
of continents —receive much support from the evidence furnished by 
the newer rocks, but are confronted with serious difficulties by the 
theory of marine origin of the Archean rocks. 
4, The vast amount of denudation which took place at the end of 
Archean timese has hardly received the notice it deserves. ‘This 
denudation is shown to have been very great by distinct kinds of 
evidence. 
Whatever the cause of production of the regional metamorphism 
of these old rocks, the fact remains that the most altered rocks are 
extensively penetrated by granitic masses. In Scandinavia the 
schists are in many places so traversed by granites that it is ex- 
ceedingly difficult to separate granite from metamorphic rock, and 
the same may be remarked in Bohemia, and judging from pub- 
lished accounts, in many other areas where Pre-Cambrian rocks 
occur. In Scotland, the great quantity of pegmatite may be judged 
from the descriptions in Dr. A. Geikie’s Text Book of Geology, 
p- 640. In North America also, granites are abundant, and whether 
or not these granites produced the metamorphism, they show that 
at the time they were intruded, the rocks which they penetrated were at 
a considerable depth below the surface. The abundance of these 
granites in Archean rocks has been explained by the age of the 
rocks, and therefore their greater likelihood of being penetrated by 
granite than newer formations. However, a great deal of this 
granite is undoubtedly of Archean age, as shown by the occurrence 
of granite pebbles in the conglomerates of the Cambrian system, 
and by the comparative rarity of granites in Cambrian rocks resting 
immediately upon Archean rocks, which are very largely penetrated 
by that rock. 
It follows that the denudation at the end of the Archean times 
was sufficient to lay bare a great quantity of granite formed at 
a considerable depth, and that such granite has been more rarely 
exposed in later times. 
The great denudation of the rocks is also shown by the extreme 
irregularity of the submerged land upon which the Cambrian rocks 
