GEOGNOSY OF THE APPALACHIANS. 453 
rather to mediate between the two, and show you somewhat of the 
two-fold aspect which geological science presents, when viewed 
respectively from the stand-points of natural history and of chem- 
istry. I can hardly do this better than in the discussion of a 
subject which for the last generation has afforded some of the most 
fascinating and perplexing problems for our geological students ; 
viz., the history of the great Appalachian mountain chain. No- 
where else in the world has a mountain system of such geographi- 
cal extent and such geological complexity been studied by such a 
number of zealous and learned investigators, and no other, it may 
be confidently asserted, has furnished such vast and important 
results to geological science. - The laws of mountain structure, as 
revealed in the Appalachians by the labors of the brothers Henry 
D. and William B. Rogers, of Lesley and of Hall, have given to 
the world the basis of a correct system of orographic geology,* 
and many of the obscure geological problems of Europe become 
plain when read in the light of our American rience. To 
discuss even in the most summary manner all of the questions 
which the theme suggests, would be a task too long for the present 
occasion, but I shall endeavor to-night in the first place to bring 
before you certain facts in the history of the physical structure, 
the mineralogy and the paleontology of the Appalachians; and 
in the second place to discuss some of the physical, chemical and 
biological conditions which have presided over the formation of 
the ancient crystalline rocks that make up so large a portion of 
our great eastern mountain system. 
I. The Geognosy of the Appalachian System. 
The age and geological relations of the crystalline stratified rocks 
of eastern North America have for a long time occupied the at- 
tention of geologists. A section across northern New York, from 
Ogdensburg on the St. Lawrence to Portland in Maine, shows the 
existence of three distinct regions of unlike crystalline schists. 
These are the Adirondacks to the west of Lake Champlain, the 
Green Mountains of Vermont, and the White Mountains of New 
Hampshire. The lithological and mineralogical differences between 
the rocks of these three regions are such as to have attracted 
the attention of some of the earlier observers. Eaton, one of the 
* Amer. Jour. Sci., II, xxx, 406. 
