O. Fisher—Physics of the Karth’s Crust. 289 
Perhaps the best feature of Mr. Fisher’s book is the skillful 
use he makes of a direct consequence or corollary of a plastic 
substratum. The slavatians and depressions of the different 
portions of the earth’s surface, he argues, are, on the theory of 
a plastic substratum, determined ydrostatic laws alone. 
Rigidity can have but slight influence upon them. - For rigidity 
is a quantity which relatively decreases as the magnitudes o 
the masses involved increase. In the continental and oceanic 
areas, in great plateaus and mountain oe rigidity is a van- 
ishing quantity, and even in individual ridges of grand pro- 
portions it probably has no great value, as compared with the 
forces which produce elevations and depressions. The ase 
mental doctrines of his book.* 
Having proceeded thus far it is somewhat surprising that 
Mr. Fisher did not advance one step farther. Elevations and 
depressions (considered as actual movements) mean one of two 
things, (1. .) Hither as quantity of matter underlying the ver- 
tically moving surface, has been increased or diminished, or 
minished. The change is either a local change of mass or a 
change of density. The contractional id aprons is sh — 
to obtain an increase of mass in elevated region 
stant mass in the depressed regions. It has iphelty “failed sae 
so must any theory of this purport fail; for later eg ousete: 
are more and more firmly establishing ‘the fact, that elevated 
regions are not regions of greater mass nor are dloprencal 
regions, regions of less mass; but the contrary. And even if 
the conclusion sought were for a moment supposed to be true, 
the continents and great plateaus, as Mr. George Darwin has 
recently shown, could not be sustained without a transcendent- 
ally rigid globe ; much less could they with a very rigid globe 
ever have been pushed up. It only remains to seek the re- 
bag solution in causes which will produce Jocal cater of 
ensitv. This dilemma is by no means sought. re appa- 
rently driven to it by the most inexorable of logical eoseial ae 
Here — question sobdivides Shall we assume that these 
we ong been convinced that this doctrine must form an importa 
of any sabe theory of the earth’s evolution. In an unpublished paper 1 hae 
surface which would follow chp: the flotation of the crust upon a liquid or highly 
plastic substratum ;—different portions of the crust being of unequal density. 
Isobarie would have been a eaaile yaks but it is preoccupied in hypsometry. 
