Re C2AUP ey) MU AVM OUN. 435 
A recapitulation of some of the topics that have been discussed is 
here presented, exhibiting briefly the origin of such of the grand fea- 
tures of the earth as depend on its gradual refrigeration and con- 
tinued contraction.* 
I. Solidification of the surface after the fluid material had lost its perfect fluidity. 
a. The change inconceivably slow, and hence the rock formed having a coarsely crystalline 
texture:—the subsequent progress of solidification beneath the crust still more gradual, and 
therefore producing at all periods of the globe a coarsely crystalline texture :—the whole the 
result of a single immeasurably prolonged operation. 
b. Hence, probably, a general uniformity in the crystalline structure, sufficient to give the 
crust apparently two directions of easiest fracture, whose mean courses are northwest-by-west 
and northeast-by-north ; yet varying much, being probably dependent to a great degree on 
the early direction of isothermal and isodynamic lines. 
c. In the progress of this cooling, commencing with its first beginning, the surface neces- 
sarily presenting large circular or elliptical areas that continued open as centres of fluidity 
and eruptive action.t Subsequently, a gradual reduction in size of these centres of igneous 
action and their frequent extinction. 
d. A boiling movement or circulation (up at centre and down around the sides) in the vast 
circular areas of igneous action, owing to escaping vapours, and dependent mainly on the tem- 
perature being greatest below at centre and least at the surface and laterally. As this cireu- 
latory or cyclosis movement occurs in material whose mineral ingredients or products differ 
in the temperature of solidification or of formation, it determines to some extent the distribu- 
tion of these mineral constituents, and of the rocks which are formed. Jn later periods, this 
cause producing a feldspathic centre to voleanic mountains having basaltic sides. 
e. As refrigeration went on, the centres of eruption becoming mostly extinct over large 
areas, and remaining still active over other areas of as great or greater extent:—for cooling, 
wherever commenced, would extend somewhat radiately from the centre where begun, (yet 
with some relation to the structural lines,) and so gradually enlarge the solidifying area and 
encroach upon the more igneous portions. 
II. Contraction, as a consequence of solidification, attended by a diminution of the 
earth’s oblateness. 
a. Rate of contraction in different parts unequal, according to the progress of refrigeration ; 
and after the formation of a crust, greater beneath the crust than in the crust itself. 
b. Contraction beneath the crust causing a subsidence of the surface. 
c. Subsidence greatest where the crust was thinnest or most yielding, and least in those 
parts which were thickest from having been first stiffened by cooling:—the large areas that 
continued to abound in igneous action therefore becoming in process of time more depressed 
than those areas that were early free (or mostly so) from such action. 
d. Subsidence of the surface progressive ; or, if the arched crust resisted subsidence, a ces- 
sation, until the tension was such as to cause fractures, and then a more or less abrupt 
subsiding. 
e. Frequent changes and oscillations in the water level, either gradual or abrupt, arising 
from the unequal progress of subsidence in different parts, and also in early periods from 
extensive igneous action. 
* For a fuller exposition of several points touched upon, we again refer to volumes ii. and iii. (ii. ser.) 
of the American Journal of Science. 
Tt Well illustrated on the surface of the moon, as also are many of the points here mentioned. (Amer. 
Jour. Sci., ii. ser. ii. 335.) 
