268 T. S. Hunt—Ristory of Pre-Cambrian Rocks 
Art. XXXIII.—The History of some Pre-Cambrian Rocks in 
America and Europe ; by T. Sterry Hunt, LL.D., F.R.S. 
{Read before the American Association for the Advancement of Science at 
Saratoga, September 1, 1879.] 
I. Inrropucrion 
OnE of the earliest distinctions in modern geology was that 
h 
them, and being in part made up of sediments derived from the 
disintegration of these, were designated Transition and nd- 
ary rocks. While the past forty years have seen great pro- 
cneee gg h the conditions of their deposition, and their geo- 
istribution and variations have been carefully in- 
rocks are portions of the first-formed crust of the planet, ae 
- | ; a 
more recent dates ; in either case, however, more or less modal 
fied by supposed metasomatic processes. By the term metas 
omatosis are conveniently designated those changes which are 
not simply internal (diagenesis), but are effected from without, 
as a result of which the chemical elements of the original rock 
are supposed to be either wholly or in part replaced by others 
from external sources (epigenesis). 
The other school, to which allusion has been made, and 
which, not less than the preceding, has helped to discourage, 1? 
