240 GEOGNOSY OF THE APPALACHIANS. [XIII. 



laws which have presided over the development of our earth, 

 and while practical geology or geognosy studies its natural 

 history as exhibited in its physical structure, its mineralogy 

 and its paleontology, it will be seen that this comprehensive 

 science is a stranger to none of the studies which are included 

 in the plan of our Association, but rather sits like a sovereign, 

 commanding in turn the services of all. 



As a student of geology, I scarcely know with which section 

 of the Association I should to-day identify myself. Let me 

 endeavor rather to mediate between the the two, and show 

 you somewhat of the twofold aspect which geological science 

 presents, when viewed respectively from the standpoints of 

 natural history and of chemistry. I can hardly do this better 

 than in the discussion of a subject which for the last genera- 

 tion has afforded some of the most fascinating and perplexing 

 problems for our geological students ; namely, the history of 

 the great Appalachian mountain chain. Nowhere else in the 

 world has a mountain system of such geographical extent and 

 such geological complexity been studied by such a number of 

 zealous and learned investigators, and no other, it may be con- 

 fidently asserted, has furnished such vast and important results 

 to geological science. The laws of mountain structure, as re- 

 vealed 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 geol- 

 ogy,* and many of the obscure geological problems of Europe 

 become plain when read in the light of our American experi- 

 ence. To discuss even in the most summary manner all of the 

 questions which the theme suggests, would be a task too l<>n^ 

 for the present occasion ; but I shall endeavor in the first place 

 to bring before you certain facts in the history of the physical 

 structure, the mineralogy, and the paleontology of the Appalachi- 

 ans ; 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. 



* Amer. Jour. Sci. (2), XXX. 406; and ante, pages 49-58. 



