392 B. WILLIS — A THEORY OF CONTINENTAL STRUCTURE 



contour of the higher continental elements. Furthermore, since these 

 elements are the masses against which the strata are pressed in the process 

 of folding, they are enveloped by the folds for that reason also. 



Thus the axial directions of folds, the Leitlinien of Suess, constitute 

 criteria for the analysis of continental structure, which are scarcely second 

 in importance to unconformities and deposits. 



9. There is abundant evidence in the broad relations of structure of 

 North America, Asia, and Europe to prove that the tangential pres- 

 sures exei-ted upon the continents proceed from the denser submarine 

 masses. The theory here presented is that these pressures are due to 

 what may be called suboceanic spread — that is, to the expansion of sub- 

 oceanic masses, sa}', 100 miles deep, at the expense of subcontinental 

 masses, in consequence of the efficiency of stress due to greater density 

 to direct movements occasioned primarily by molecular or mass changes 

 under varying conditions of temperature and pressure. The general result 

 is plastic* flow in rigid and solid rock masses. It is further held that in 

 the great suboceanic regions such flow is a persistent condition, to which 

 we may ascribe those accumulated stresses that have sufficed to produce 

 the occasional pronounced effects of diastrophism.f 



10. It is held to be demonstrable, on the evidence of sequences and 

 volumes of sediments, that the tendencies toward diverse vertical dis- 

 placement of tlie continental masses have been effective in producing 

 movement only during relatively short epochs between long intervals. 

 Between the epochs of active displacement the tendencies toward 

 movement were relatively ineffective. Hence we may recognize cycles 

 of diastrophism, each one of which comprises {a) a comparatively brief 

 epoch of erogenic and epeirogenic activity, which results in elevated 

 lands and restricted mediterraneans, energetic erosion and voluminous 

 terrigenous sediments, and climatic and faunal diversities; and (6) a 

 comparatively long period of continental stability, which results in exten- 

 sive peneplanation, meager terrigenous sediments and general marine 

 deposits, extended epicontinental seas, and climatic and faunal uni- 

 formity. J 



11. The critical times which bring out continental structure are the 



*A common connotation of the term plastic, namely, soft or softened, is explicitly ex- 

 cluded liere. Plastic is used to describe tlie mode, not tlie ease, of motion. 



tBailey Willis : Research in China, vol. ii, Systematic Geology, chapter viii. Publica- 

 tion no. 54, Carnegie Institution of Washingtog, 1907. 



t The evidence of periodicity in earth movements and the effects of periodicity on 

 climates and faunas are broadly and exhaustively set forth in the works of the author 

 of the theory, T. C. Chamberlin. See Chicago Journal of Geology ; Manual of Geology, 

 Chamberlin and Salisbury, and Fundamental Problems of Geology, in year books and 

 publications of the Carnegie Institution of Washington. 



