252 J.D. Dana on the origin of the Earth’s Features. 
Art. XL.—Appendix to Article XXX, On the Origin of some of 
the Harth’s Features; by JAMES D. Dana. 
On page 210 I have made but a bare allusion to the question 
of the heat required in metamorphism. . Vose dispenses 
with heat altogether, except what may be incidental to compres- 
sion. And Professor Hall regards it as of secondary importance, 
or not absolutely necessary (see our citation on page 207), and 
attributes the little extraneous heat that may be present and 
operative—probably, he says ‘‘ not much above that of boiling 
water” (Pal., vol. iti, p. 77)—to the sinking of the thickening 
deposits to a level ‘‘where the surrounding temperature was 
higher ;” higher, that is, on the principle, first suggested by Her- 
schel, of the rising of the isothermal planes within the earth’s 
crust in concordance with increase of thickness through super- 
ficial deposits; the isothermal plane of 100°, for example, being 
within a certain distance of the surface of the crust in a given re- 
gion, and rising as the surface rises by new accumulations above. 
The correctness of Herschel’s principle cannot be doubted. 
But the question of its actual agency in ordinary metamorphism 
must be decided by an appeal to facts; and on this point I would 
here present a few facts for consideration. 
The numbers and boldness of the flexures in the rocks of most 
metamorphic regions have always seemed to me to bear against 
the view that the heat causing the change had ascended by the 
very quiet method recognized in this theory. For the heat, thus 
slowly creeping upward, a few inches, feet, or yards in a cen- 
tury, should produce the change with little disturbance in the 
mass, and leave the beds nearly or quite horizontal: a condition 
very unlike that actually found in nature. The region of the 
thickened accumulations is also necessarily, as I have said, one 
of strengthened crust, under the petite ley poibeses and dis- 
