thr extension of normal eaulting. 61 



those of the coastal plain ; as in the coastal plain, each formation is cx- 

 plical)le only as a record of submergence ; and each unconformity of tlie 

 usual type is expHcable onl}^ as a record of emergence, with sculpturino- 

 by rain and rivers. Throughout this area of 2,250,000 squares miles the 

 strata are occasionally divided b}^ faults hading to the downthrow, and 

 therefore indicating horizontal extension. The formations and uncon- 

 formities are many, and collectively they constitute a complex record of 

 continental oscillation, the amplitude of which is to be measured by 

 thousands of feet or even (in the borders of the mountainous regions) by 

 vertical miles; yet throughout this area of three- fourths of our territor}^ 

 the record gives only subordinate indication of horizontal movement in 

 tlie earthcrust. The great, oft-rei)eated, and long-continued movement 

 was vertical, the horizontal movement was relatively slight and, so far as 

 it goes, indicates extension. 



The case of the United States is one of many. In ever}^ great conti- 

 nent on the globe there is a relatively small area of mountains composed 

 of strongly deformed rocks, and a relatively large area in which the strata 

 furnish a record of vertical oscillations of wide amplitude with little hori- 

 zontal movement. One-fifth, or perhaps one-fourth, of the land area of 

 the globe is mountainous and deformed ; the larger fraction is non-moun- 

 tainous and not deformed, at least superficially, i. e., in normal condition; 

 and thus the normal record of geology is essentially one of vertical move- 

 ment in the earthcrust. The inferences involved in interpreting the 

 record are direct, and hence subject to little error; the formations are 

 essentially like those observed in process of accumulation, the uncon- 

 formities essentially like the land-forms seen in process of sculi)turing, 

 and the coastal undulations are essentially like those measured in Hol- 

 land ; even the inference of stratic extension from normal faults is a 

 simple application of geometry. 



ESSENTIAL CHARACTERS OF EARTH MOVEMENTS. 



During recent years orogeny, or mountain-raising, is discriminated 

 from epeirogeny, or continent-lifting, by many geologists. Orogenv is 

 relatively local, temi)orary, exceptional; epeirogeny is relatively Avide- 

 spread, constant, general — ?. c, the former is the aberrant or abnormal, the 

 latter the characteristic or normal. Moreover, the processes of orogeny 

 (exce[)t of the subnormal Great basin tyjje) transcend experience, and 

 are not inferred directly from observation but only from products l)y 

 indirect analogy, while the processes of epeirogeny are subjects-matter 

 of exijcricnce, and those not seen are inferred directly from observed 

 proces.scs. Accordingly it is legitimate, albeit unusual (as set forth 

 later), to confine attention to the normal ;ind the relativc'ly definit(! in 



