896 THE AMERICAN NATURALIST. [Vou. XXXII. 
Appalachians, are evidenced in gentle undulations, with the 
exception of the Cincinnati axis, which is more important, 
extending from Lake Ontario to Alabama, and is the last or 
most western of those parallel to the Appalachian chain.” 
It does not appear clearly that the physical consequences of 
his views were ever elaborated in his own mind, or that the 
thermal features of the problem, as somewhat narrowly pre- 
sented by T. Mellard Reade, were studied. Indeed, there is 
discernible in Hall’s writing a shrinking from the reference of 
mountain topography to dynamical agencies, but a quick re- 
sponse of interest to their indications of sculpture by erosion. 
If we might venture a pleasantry, we should say that if Pro- 
fessor Hall, as deus ex machina, had been permitted to have his 
own way, the Catskill rather than the Appalachian type of 
mountains would have been most widely distributed over the 
earth’s surface. 
Certain metamorphism and folding were recorded, and the 
contrasted phases of mountain-making exhibited in the Cats- 
kills and the Appalachians pointed out, but the metamorphism 
and folding were referred tothe consequences of wezgh¢ and not 
to crustal shortening. Here again Hall was quick to respond 
to contemporaneous investigation. He recognized that the 
facts of metamorphism did not require an enormous heat, and 
hinted at those hydrothermal processes which lithologists now 
find so active and efficient in producing mineral alteration. He 
says : “We must therefore look to some other agency than heat 
for the production of the phenomena witnessed ; and it seems 
that the prime cause must have existed within the material 
itself, and that the entire change is due to motion, or fermen- 
tation and pressure, aided by a moderate increase of tempera- 
ture, producing chemical change.” 
The view of mountain-making propounded by Hall was an 
illustration of common sense illumined by thought and obser- 
vation. Yet it was in the nature of a revelation. Le Conte 
has told us “the idea was so entirely new, so utterly opposed 
to prevailing views, that it was wholly incomprehensible even to 
the foremost geologists. There was no place in the geological 
mind where it could find lodgment. It was curious to observe 
