258 Willis and Hayes — Conditions of 



the conditions which gave rise to the great system of over- 

 thrusts in the Appalachian province are there discussed. 



But the article by Dr. E. A. Smith in this Journal for 

 April, 1893, on Underthrust Folds and Faults, suggests the 

 desirability of briefly presenting our views through the same 

 medium. If the normal Appalachian overthrusts and the so- 

 called " underthrusts " are produced by the same mechanical 

 forces slightly modified by local conditions it seems unneces- 

 sary as well as undesirable to create a new species for the latter 

 in the systematic classification of Appalachian structural forms. 



First will be stated the antecedent conditions for the devel- 

 opment of Appalachian structure ; second, the mechanics of 

 stepfolds and thrust-faults and repeated parallel folds or faults ; 

 third, application of the theoretical conclusions will be made 

 to the actually observed Appalachian structure, and finally the 

 direction from which the compressing force acted will be 

 briefly discussed. 



Sedimentation and initial dips. — The source of the greater 

 part of Paleozoic sediments of the central and southern Appa 

 lachians is generally assumed to be a continental area lying 

 east and southeast of the interior area of deposition. This 

 was doubtless the original source of the material but the later 

 formations are probably largely made up of re-deposited mate- 

 rial eroded from those earlier formed. From the observed 

 processes of sedimentation it is known that mechanical sedi- 

 ment from the land is deposited over a zone parallel to the 

 shore and extending to a greater or less distance out to sea. 

 But under the usual conditions of tides and undertow the 

 greatest thickness of the deposit will be in a narrow belt along 

 shore and will gradually lessen seaward. Throughout great 

 masses of sediment it is frequently observed that the layers 

 bear the rippled marked surfaces formed in shallow water, com- 

 pelling the conclusion that subsidence went on at a rate equal 

 to that of deposition. The subsidence is an argument for 

 isostatic adjustment to shifting load. It is also an argument, 

 on the assumption of a general cause of subsidence other than 

 isostasy, that the effects of such cause may be localized and 

 the subsidence accelerated by loading. Or it might be argued 

 that any force acting vertically may deepen the water and that 

 off shore currents expanding in deeper water and therefore 

 slowing there would deposit heavily upon the subsiding area. 

 Whatever view be taken of the process the fact remains that 

 subsidence has frequently gone on at a rate approximately 

 equal to that of accumulation of strata. The cross section of 

 such a deposit is that of a bi-convex lens. One edge rests 

 against the shore from which it thickens rapidly and then 

 thins gradually seaward. The lower surface of this lens is 



