TOPOGRAPHICAL FEATURES. 427 
cal currents on the position of crystals in process of formation, is 
another fact established by modern science ;* and the constant circu- 
lation of these currents about the earth has been ascertained, as well 
as the direction of isodynamic lines. As solidification would take 
place with extreme slowness after the first crusting of the surface 
began, and would continue afterward at a rate inconceivably more 
slow, the circumstances would be favourable through all ages, subse- 
quent to the first step in the process, for a coarse crystallization of the 
material below, and for the general operation of electric currents. 
It was first shown by Brewster that the isodynamic lines of the 
globe correspond with isothermal lines. Within a recent period, this 
relation of heat and magnetism has been mathematically investigated 
by Prof. William A. Norton.t The courses of electric currents, or rather 
the lines of equal intensity, would consequently be lines of equal heat 
or equal cooling, and therefore of equal tension asa result of the contrac- 
tion attending refrigeration. ‘Tension, heat, and electricity, therefore, 
would be at first combined in producing a common result, and we 
cannot doubt that some degree of structure would necessarily be thus 
impressed upon the cooling crust, and a structure analogous to that 
in the crystalline rocks of the surface. These crystalline surface- 
rocks illustrate well the result, although they must be viewed as dis- 
tinct to a great extent from the cooled material which by a single 
long-continued operation has been gradually solidifying beneath the 
surface: and they should not necessarily conform to the latter in the 
direction of cleavage. ‘The two transverse cleavages of granite due 
to its feldspar are well known; and it is also a fact, that feldspar is 
by far the most abundant mineral in igneous rocks. ‘There is then 
avery probable cause before us for the structure which is shown 
to pertain to the solid material of our globe. 
There is certainly a striking coincidence between the trends of 
many of the island ranges of the globe, and a chart of isodynamic lines, 
as may be seen by comparing our chart with the chart of magnetic 
and nutation, that “‘ the minimum thickness of the crust of the globe, which can be deemed 
consistent with the observed amount of pressure, cannot be less than one-fourth of the 
earth’s radius.”—Researches in Phys. Geog., in Trans, Roy. Soc. London, for 1839, 
p. 381, for 1840, p. 198, and for 1842, p. 43. 
* R. Hunt, L. E. & D. Philosophical Magazine and Journal of Science, Jan. 1846, 
page 1; American Journal of Science, ii. Ser. ii. 116.—R. W. Fox, Report of the Poly- 
technic Society of Cornwall for 1837, pp. 20, 21, and 68, 69. 
+ Amer, Jour. Sci. ii. Ser. iv. 1, 207. 
