468 J. LeConte—Formation of the 
on the subject of the aqueo-igneous fusion of deeply buried 
sediments. 
e accumulation of sediment, as first shown by Babbage, 
and afterward by Sir John Herschell, necessarily produces a 
rise of the geo-isotherms and an invasion of the sediments by 
the interior heat of the earth. From this cause alone, taking 
the increase of interior heat at 1° for every 58 feet, or about 
90° per mile, and adding the mean surface temperature (60°), 
the lower portion of 10,000 feet of sediments must be at a tem- 
tae of about 230°, and of sediments 40,000 feet thick, 
ike those of the Appalachian chain, must be nearly 800° F. 
Even the former moderate temperature, long continued in the 
presence of the included water of the sediments, would be 
sufficient to produce incipient change—at least lithification, if 
not metamorphism. In fact, lithification of sediments wi 
probably take place under heavy pressure even at ordinary 
temperature, but is no doubt hastened by high temperature. 
The latter temperature of 800° is certainly sufficient to produce 
not only metamorphism, but aqueo-igneous pastiness, OF 
even complete aqueo-igneous fusion. With a small quantity 
of alkali in the included water of such sediments, all these 
ably continues during this process. Finally, this softening deler- 
mines a line of yielding to horizontal pressure, and a consequent 
up-swelling of the line into a chain. us are accounted for, 
first, the subsidence, then the subsequent upheaval, and also the 
metamorphism of the lower strata so universal in great mountain 
chains. By this view, of course, the exposure of the metamor 
a rocks on the surface is the result of subsequent erosion. 
ven the granite axis, I believe, in most cases, is but the lower- 
most, and therefore the most changed portion of the squeezed 
mass, exposed by subsequent erosion; although it is by n° 
means impossible that in some cases the granite may he squee 
out as a pasty mass through a rupture at the top of the swelling 
mass of strata. 
The theory, as will be observed, strongly inclines toward 
the metamorphic origin of granite, but does not require it. 
_ For there is nothing to hinder the aqueo-igneous fusion of ap 
_ original granite crust by the accumulation of sediments upo? 
it, and the consequent yielding of the crust along the line of 
accumulation,” ; 
Soe 
