2 THE STRUCTURE OF THE EARTH. 



Petrology treats of" the origin, history, physical features, mode of occur 

 reuce, and relations of rock masses. 



Liihology is essentially an in-door or cabinet and laboratory science; \vliile 

 pelrolof/y is exclusively a field study. The former needs for its pursuit hand 

 specimens only; for the latter we must have the rocks in situ. 



Petrography I define as that branch of science which cmbi-aces both 

 lithology and petrology. It includes everything that pertains to the origin, 

 formation, occurrence, alteration, history, relations, structure, and classifica- 

 tion of rocks as such. It is the essential union of field and laboratory studj^ 



So far as possible my work has been carried on according to petro- 

 graphical rather than ordinary lithological methods, and with the belief 

 that field evidence is stronger than any laboratory evidence can be in 

 all matters relating to the origin of rocks. 



The fxcts developed by petrographical study seem to me to demand for 

 their explanation a former liquid condition of this globe, and the admission 

 that all rocks, not of organic origin, now forming a portion of the earth's 

 crust, are the results either of that molten condition, or of the action of 

 atmospheric and hydrous agencies upon the formerly liquid material. The 

 belief in the former fluid condition is in accord with the demands of geology 

 and the results of physical and astronomical research; for it seems proper to 

 hold, that, as is the present physical condition of the nebulae, stars, sun, 

 planets, and satellites, so was, or is, or will be, this earth. Indeed the 

 various phenomena with which we are concerned seem to be but the con- 

 comitants and results of the passage of this earth from its active condition, 

 as a hot fluid mass, towards a cold, inert, and passive state. Is it not our 

 part to study matter in its present transitory stage, and from the facts thus 

 gathered to reconstruct as far as possible its past history and infer its future 

 course ? To me the beginnings, the various transitory stages, and signs of 

 what the end will be, are apparent in the rocks ; and the effort of the classi- 

 fication here employed is to give voice to these changes, or to the unstable- 

 ness of the rock constituents, while the classifications of others appear in 

 general to be based upon the assumed stability of the rock constituents, — 

 that is, they assume that as the rocks now are so they were, and always 

 will be. 



I am unable to explain the facts obtained in the petrographical study of 

 the rocks except on the supposition that the eruptive rocks of all kinds 

 came from the interior part of the earth, and from below the sedimentary 



