work: of the present 115 



bringing the rocks from some original position into those which they 

 now occupy, and the study of the effective forces. 



New lines of theory to account for oceanic basins and continental 

 plateaus, for thalassic deeps and mountain ranges, for rock folding, 

 igneous intrusions, and vulcanism are opening before the student. 

 From the conception of a heterogeneous earth comes the idea of isostasy, 

 which, in its largest statement, is the theory that heavier masses beneath 

 ocean basins are in balance with lighter masses under continents. The 

 facts of folding are now understood to be of very superficial character 

 considered relatively to the earth's radius, and any particular anticlinal 

 or synclinal structure may be studied as a simple problem in mechanics 

 involving certain materials, loads, and stresses. The sequence of sedi- 

 ments in any marine basin is the record of changing physical conditions 

 along the shores and on the adjacent land. The development of a shore 

 from youth to age, the growth of coastal plains, and the general topo- 

 graphic phases which passed on the adjacent lands are to be read in the 

 rocks. Through such keys we shall decipher the history of the physical 

 geography of the earth and the succession of mountain growths, and we 

 shall approach the greater problem of continental growth. 



Structural studies are now pursued with a better understanding of 

 the mechanics of mountain growth, and through physiography and stra- 

 tigraphy the history of deformation, past and present, may be made out. 

 The science of structural geology ma}^ be said to be in its youth, with a 

 future of great promise before it. 



In the interaction between applied geology and pure geologic science 

 lies the charm and the recompense of every-day routine geologic work. 

 For the sake of future generalizations and for the sake of indicating the 

 distribution of formations having economic value, the geologist performs 

 a great deal of routine labor — observing phenomena of familiar kinds, 

 grouping them in well known categories, and making a record for future 

 use, chiefly by others. He describes rock masses, with measurements 

 of thickness, extent, and dip. He sorts rock specimens, giving to the 

 familiar familiar names and describing the characters of the novel. He 

 sorts collections of fossils, recognizing species already known and de- 

 scribing such as are new. In performing these various duties he uses 

 well established methods, and merely applies to new material the known 

 principles of the science. Such work is necessarily monotonous, and 

 the active mind would soon lose interest and its operation become per- 

 functory were it not for the possibility of discovering new principles. 

 But ever and anon a fact is found for which the science has provided no 

 pigeon-hole — a phenomenon which is not explained by any known 



XVII— Bull. Geol. Soc. Am., Vol. 13, 1901 



