96 J. D. Dana on the Origin of Continents. 
3. Fissure 
A. Bjection of igneous matter, at times, through fissures.* 
5. Upheaval along a line of fissure, the surface adjoining being 
more or less raise 
6. Upliftings and foldings from lateral pressure.—An are of the 
exterior surface being greater than any corresponding arc below 
the surface, a depression of the hardened exterior, produced by 
the cooling beneath, would in some instances cause lateral dis- 
placements. 
7. An unequal rate of subsidence over given areas in different 
periods.—Contraction tends to occasion a strain upon the cooled 
and unyielding exterior, accompanied generally by a consequent 
diminished rate of subsidence, or a cessation of it. This strain 
increases till it results i in fractures ; and following this crisis, sub- 
within or without the area; and at the time of fissuring, there 
might be other upheavals. It follows, hence, that— 
a. There would be prolonged intermissions in the subsidence 
of given areas; and this must have been the fact throughout the 
history of the § 
There reat fae been oscillations in the land as com- 
pared with a water level, the water at times rising gradually 
over land that, during a previous period, had emerged; and the 
reverse. 
c. There might be in the same epoch, under such circumstan- 
ces, an unequal retreat of the ocean from the coasts of different 
oe ntinents, or a rise in one place and a retreat in others: for the 
ges by contraction are supposed to have been every where in 
ous t the same time, and throughout different in character 
and extent. 
d. Changes of level may _ some cases have been gradual, 
and in other cases parorysmal ; for the opening of large fissures 
would often be of the latter iti 
bi In an elliptical area of contraction, there will be two sys- 
of fissures at right angles with one another, as follows from 
ring cdlénilating of Win. Hopkins, Esq.t But if the area is bounded 
on one side by a region participating but little in the ee 
the effects would be most decided on the borders of such a 
gion ; and they would consist in extensive fissures ranging sidng 
* Prévost argnes that all oepsem of igneous matter have arisen from the col- 
Japsing of the surface upon the fluid of ve interior, which is thereby pressed out. 
This is a pro cable effect of — contraction going on, though it seems to be ex- 
© include with it all the eruptions of volcanoes. 
t Trans. Camb. Phil. Soc., vii, 22. 
