108 J. LeConte—Structure and Origin of Mountains. 
what property do they thus move? Forbes answers, by a 
property of viscosity ; Tyndall answers, by fracture, change of 
position and regelation. These are Physical theories. Observe 
again: the formal theory is independent of the physical and 
forms its basis; but not vice versa. Now in both these cases 
it will be observed that it is the formal theory which is most 
important to the geologist. That slaty cleavage is produced by 
mashing together horizontally and upswelling vertically, or that 
glaciers move in the manner of a stream, is of immense impor- 
tance to the geologist; but the molecular cause of either is of 
great interest only to the physicist. When slaty cleavage was 
cussion. 
Now on the question of mountain origin we find the same 
two kinds of theories: That mountain ranges are formed b 
horizontal pressure crushing rock masses together in that 
direction and upswelling them vertically, is certain; and that 
this horizontal pressure is due to interior contraction of the 
earth is almost equally certain. This is the formal theory. 
But still the question remains: What are the physical causes 
of interior contraction? The discussion of this is the physical 
theory. The former we have shown is nearly perfect; the 
physicists for further discussion. But we must insist that the 
physicist shall make the formal ‘ans already established by 
the geologist the basis of his discussion. But observe again sic? 
it is the formal theory which is of the greatest and most im 
diate importance to the geologist, rio the physical haere 
may be the most so to the physicist. 
The two physical objections which I have just taken up and, 
I hope in part at least, answered, are brought forward by Rev 
O. Fisher* and Captain C. E. Dutton. + They are by far the 
most serious. But there are other minor objections advanced 
by Captain Dutton which I must, at least briefly, notice. 
e are concerned in this paper only with the origin of 
mountains ; but in my paper on “A Theory of the formation 
of the greater features of the Earth surface,” I discussed also 
the origin of continents. I attributed those greatest inequalities 
constituting continental surfaces and ocean bottoms to unequal 
radial contraction or a_ secular deformation, by cooling, of a 
heterogeneous earth. The same idea had been previously 
*“On the Inequalities of the Earth Surface,” &¢., Cambridge Phil. Trans., vol. 
xii, part 2d, Dec., a a ee: Phil. Trans., vol. xii, part 2d, Feb., 1875. 
+ “Cri itical Obse on Theories of the Earth Physical Evolution,” this 
Journal, viii, 113, 1874s. Peat Monthly, May, 1876. 
| 
| 
