306 E. A. Smith — Underthrust Folds and Faults. 



very reverse of what we commonly see in the Appalachian 

 region, was to be explained on supposition that in the folding, 

 instead of the arch limb of the fold being, as is usually the 

 case, thrust up over the trough-limb, and lapped over toward 

 the northwest, the trough-limb was thrust under the crest or 

 arch-limb, the effect being the same as if, with a force acting 

 from the northwest, the fold had been lapped over in the usual 

 way, i. e. in the direction towards which the force was exerted, 

 or towards the southeast. Inasmuch as we could not easily 

 account for a force acting from the northwest, and since this 

 class of folds has demonstrably a common origin with the 

 folds of the regular Appalachian type, it has seemed to me 

 that the above explanation is the true one. 



In the Appendix to Squires's Report on the Cahaba Coal 

 Field, I have discussed this type of structure, and have spoken 

 of the fault as a reversed thrust fault in contradistinction to 

 the thrust fault of the normal type which so greatly prevails 

 in the Appalachian region. 



I have since thought that another name, which would more 

 clearly point to the mode of formation, would be preferable, 

 and therefore suggest the following terms, under fault, fault- 

 ed under fold, underthrust fault, as applicable to this type of 

 structure. These terms correspond with those used by Mar- 

 gerie and Heim in describing the common type of thrust fault. 

 In speaking of the causes that determine the direction towards 

 which a fold is overlapped, these authors say,* " Der Sinn, in 

 welchem eine Falte geneigt oder iiberscnoben ist, hangt 

 nicht direct von der absoluten geographischen Richtung des 

 einseitigen Schubes in diesem Stuck Erdrinde ab. Verschie- 

 dene locale Umstande bedingen die Richtung der Faltennei- 

 gung. Wenn, z. B. die Basis beiderseits einer beginnenden 

 Falte ungleich hoch ist, so ist die Falte geneigt, sich nach den 

 tief eren Seite ueberzulegen .... Wenn die Falten einer Zone 

 nicht mehr aufrecht sind, findet man fast immer die grosse 

 Mehrzahl derselben in gleicher Richtung geneigt oder iiber- 

 gelegt." 



Fie. 1. Overthrust. Fig. 2. Underthrust. 



The sketches given, in which the arrows show the direction 

 of the folding force, will serve both to illustrate the remarks 

 of Margerie and Heim just quoted, and to show that the two 

 types of structure under discussion may result from the action 

 of the same- force acting under slightly different conditions. 



-Die Dislocationen der Erdrinde, p. 81. 



