12 J. D. Dana— Results of the Earth's Contraction. 
4. According to the above, the solid part of the globe con- 
sists, as regards origin, of three parts. 
a. The central mass, consolidated by pressure; the solidifica- 
tion centrifugal, or from the center outward. 
b. The crust proper, consolidated by cooling; the solidifica- 
tion centripetal, or from the surface inward. 
c. The outer crust, or superficial coatings—the supercrust— 
made chiefly by the working over and elaborating of the mate- 
rial of the surface through external agencies, aided by the 
ever-acting lateral force from contraction, and including all 
terranes from the Archeean upward. 
5. As to the thickness of the viscous layer and the overlying 
crust, or the vif of the later under-crust seas, I have nothing 
_ The under-crust fire-seas would have had their heat from 
time to time supplemented through the movements of the crust. 
pared with the earth’s surface, that this cause could not have 
prevented a gradual narrowing of their limits with the progress- 
ing refrigeration. But even after the general union of the crust 
and nucleus, giving the earth trap-like “ rigidity,” had taken 
A final word on Mountain-making. From the above we learn 
that. in the work of mountain-making in eastern North Amer- 
ica, there was first the commencing and progressing geanticlinal 
on the sea-border; and, as a concomitant effect of the latera 
pressure, a parallel geosynclinal farther west, along the border 
of the continent. Concurrently, the deepening trough of the 
geosynclinal was kept filled to the water level, or nearly, by 
sedimentary accumulations, until these had become seven miles 
in thickness; and, as a consequence, the lines of equal tempera- 
ture (isogeotherms) in the crust beneath gradually rose upward 
seven miles; and further, the geosynclinal crust, owing to this 
rising of heat from below, lost part of its thickness by a melting 
off of an under portion, and also part of its strength up to a 
higher level by the softening action of the heat, while it re- 
ceived, as the only compensation for the loss of thickness, the 
addition of half-consolidated sediments above. Finally, the geo- 
synclinal region, owing to its position against the more stable 
