F. A. Smith— Underthrust Folds and Faults. 305 



It is quite conceivable that nepheline crystals, with more or 

 less feldspar might do the same, and subsequently alter to 

 analcite. 



On the other hand leucitic rocks in Brazil and elsewhere as 

 cited by Hussak, present analogous phenomena. Leucite often 

 tends to attract tangential rims and inclusions. It is, however, a 

 mineral of comparatively restricted occurrence and although 

 recently demonstrated in two new localities, Arkansas and 

 Brazil, in association with elseolite-syenites, the probabilities 

 are against it until unquestionably shown. 



In the writer's judgment the question cannot be settled 

 without the discovery of perfectly fresh material either of 

 nepheline or leucite in the spheroids, and pending this, that 

 the determination of the rock as a leucite-tephrite is prema- 

 ture, and should only be suggested as a future possibility. But 

 if it should be demonstrated in New Jersey in association with 

 the elseolite-syenite, it would form a most interesting addition 

 to the geology of the region. 



Geological Department, Columbia College. 



Akt. XXXVI. — Underthrust Folds and Faults / by 

 Eugene A. Smith, University of Alabama. 



In the normal Appalachian fold, it has been repeatedly 

 pointed out that one side of the fold is much steeper than the 

 other, and that these folds have been produced by the action 

 of a force coming from the direction of the Atlantic, and 

 hence from the east and southeast, is very generally admitted. 

 In the great majority of cases these folds have been pushed or 

 lapped over towards the northwest, so that the more gently 

 sloping strata dip toward the southeast, while those with the 

 steeper dip are turned towards the northwest. So also when a 

 fault occurs along the steeper side of the fold, the strata on the 

 soiTtheast of the fault are thrust up over those on the northwest 

 side. Such faulted folds have been called by a variety of 

 names; overfaidis, faulted ove? folds, etc., and the general 

 term overthrust faults would also apply to these, especially 

 where the overriding has been very considerable. 



A few instances have come under observation in Alabama, 

 and are mentioned also in other States, where the steeper side 

 of the fold lies to the southeast, and the more gently inclined 

 side to the northwest, and where the fold has been faulted, the 

 gently inclined northwest dipping strata seem to have been 

 thrust up over the more highly inclined beds to the southeast. 



It was suggested by Mr. A. M. Gibson of the Alabama Geo- 

 logical Survey some years ago, that this peculiar structure, the 



