242 The American Geologist. April, 1892 
Noy, 2, 1891,* the manner in which the internal forces of the 
earth are thus unlocked. 
When we come to test physical theories by geological facts it 
is impossible to ignore the intimate relation that exists between 
sedimentation and mountain formation. It is quite unnecessary 
for me to dwell upon this in an American publication, for it is to 
the lasting credit of American geologists that they were the first 
to establish the fact, and no theory which does not take it into 
account as a first principle will ever be likely to establish itself 
as a reasonable explanation. This has been seen by many 
and from the time of Sir John Herschel down to the present, 
loading by sedimentation and unloading by denudation have been 
considered more or less a vera causa. It does not need deep 
thinking however to see that this can be but a partial explanation. 
It is a machinery that must in the absence of some other opposite 
force evidently come to an end—it must run down. Some of the 
geologists of the Indian Survey account for the supposed con- 
tinued rising of the Himalayas in a similar way by the denudation 
of the mountains and the laying down of sediment on their 
flanks and onthe Gangetic plain. At the best, whatever value we 
may be inclined to attach to the explanation, the Himalayas can- 
not have or/ginated in this manner. It is nota theory of ‘‘origin” 
but of ‘‘matntenance’ and the lateral pressure that it provides, 
is, compared with any form of the contraction or expansion 
theories, almost zero. 
Mr. Fisher has lately introduced the conception of a fluid zone 
subject to convection currents. These convection currents flow- 
ing from under the crust under the great oceans are supposed to 
drag the crust towards the continents and to produce lateral pres- 
sure and mountain folding on their margins. Granting all the 
hypothetical conditions required—and this is granting a 
great deal—it is difficult to conceive how convection currents 
which can only originate from differences of specific gravity in 
the fluid itself, due to differences of temperature, could produce 
the necessary force, and still less, act continuously in certain 
(lirections through all the great time occupied in the building of 
i mountain range. A mountain range is too permanent a feature 
of the earth's surface to have originated or been maintained in 
this manner. 
*Originally published in the Phil. Mag. 
