286 O. Fisher 
Physics of the Earth’s Crust. 
rest in the crust would be approximately the position of hydro- 
static equilibrium. r. Fisher assumes this plastic substratum 
henceforward through the remainder of his work. 
The subsequent chapters are devoted chiefly to a considera- 
tion of the consequences which would flow from the following 
postulates, both of which Mr. Fisher regards as being forced 
upon our conviction by the nature of the facts to be explained. 
~ 1st. That lateral compression has acted upon a grand scale in 
developing the earth’s physical features; 2d, that the earth has 
an inflexible crust resting upon a liquid or plastic substratum. 
It would be impossible in so brief a notice to do justice to the 
many ingenious, suggestive and valuable ideas he throws out 
in this discussion and only the most striking ones can be 
alluded to. ; 
Mr. Fisher inclines to the following explanation of the origin 
of mountains. He recognizes difficulties in it but it avoids 
more difficulties than any other he can think of. Granting a 
plastic substratum of somewhat greater density than the crust, 
it is possible that the compression of the strata (from whatso- 
ever cause arising) may be localized in a narrow belt or zone. 
At this disturbed tract, as he calls it, where the yielding takes 
place, the crust becomes thickened greatly. But its position of 
rest must observe the law of hydrostatic equilibrium or simple 
flotation. If,by means of compression the amount of lighter 
crust-matter is increased in any locality it displaces denser mat- 
ter in the plastic substratum. The height to which the surface 
of the disturbed tract will rise above the mean level will be 
greater than the bulge upward. Mr. Fisher’s great difficulty 
