2 THE STEUCTUEE OF THE EAETH. 



Petrology treats of the origin, history, phj^sical features, mode of occur 

 rence, and rehitions of rock masses. 



Lithologi/ is essentially an in-door or cabinet and laboratory science ; while 

 petrology is exclusively a field study. The former needs for its pursuit hand 

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



Petrography I define as that branch of science which embraces both 

 Mthology 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 2)etro- 

 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 oritjirin of rocks. 



The facts 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 oi: 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 pro23er 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 eartli. Indeed the 

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

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

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

 2:>art 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 begiimings, 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 bo 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 stud}^ of 

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

 came from tlje interior part of the earth, and from Ijelow the sedimentary 



