Origin of the Grand Outline Features of the Earth. 395 
mainly caused by tension consequent on a diminution of the 
earth’s oblateness. 
Whatever the origin, there can be no doubt of the fact, that a 
kind of cleavage structure, or, at least, a capability of fracturing 
most easily in two directions, was given the crust during its form- 
ation, and’that such a structure has influenced the direction of 
the lines of fissures that have since taken place. And while there 
is evidence of this structure, there is proof that the rupturing 
force often acted obliquely to the planes of easiest fracture, caus- 
ing deviations from straight lines in the long ranges. e next 
question is with reference to this rupturing force. 
Contraction is a known dynamical cause that must have begun 
with the beginning of refrigeration; and it is hence essential to 
consider how far it meets the various facts in view. In the the- 
ory of mountain ranges, by Elie de Beaumont, this agency is ap- 
- The effects of contraction have been illustrated elsewhere in 
this volume. A prime feature in the operation of cooling, infiu- 
encing all the results proceeding from it, depends on the tendency 
of heat to spread itself circularly, or to diminish circularly, around 
acentre. This cause gives a circular form to pools of lava, and 
they retain this form as they cool. 'The great crater areas of the 
moon, several hundred miles in diameter, illustrate it; and this 
size is no necessary limit to their extent. Ina cooling globe 
there would therefore be necessarily such vast circular or elliptical 
areas. Here then we perceive a cause modifying all the results 
of cooling ; and we observe that throughout all ages there must 
therefore have been some reference to such circular or elliptical 
areas in contraction ; and especially, to aggregations of such areas, 
which also would be more or less curvilinear in outline, and 
would act as a whole in the progressive subsidence. ; 
The force of tension in the crust from contraction beneath, 
ponded ; otherwise a series of rents should result having a direc- 
tion of range different from the direction of the line of structure. 
The peculiarities of fissures, which have been explained, the 
€ receding,” or “advancing,” or “ continued” series of parallel 
courses, and the curved directions, are therefore necessary effects 
the cause appealed to. Curved as well as straight ranges, 
ht therefore to characterize the grand features of the globe. 
e important generalizations of Mr. Hopkins with regard to 
the direction of fractures and the necessary dependence of two 
