MUSEUM OF COMPARATIVE ZOOLOGY. 325 



dislocation would probably be called true unconformity if it had taken 

 place and been laid bare just at the junction of this formation with the 

 Hudson strata below it; but happening twenty feet higher, it can be 

 referred to its true cause. Secondly, we may note the effect of internal 

 disturbance in the limestone, shown by the breaking up of some of its 

 tine layers (fig. IG) : this must be referred to the epoch of general fold- 

 in<z, for examples can be found of all sizes up to a coarse brecciated mass 

 several feet thick near the fault-unconformity ; it is therefore an example 

 of what Heim calls folding with fracture, as distinguished from folding 

 by bending; and is explained by him as a folding that took place under 

 a pressure of superincumbent rocks insufficient to cause plasticity in the 

 brittle strata.* This fact may some day yield a measure of how many 

 hundred or thousand feet of rock were not over the Waterlime when it 

 was folded, and so lead to a conclusion respecting the. former eastward 

 extension of the Hamilton and Catskill sandstones. 



Another conclusion may be noted : the strata showing these crowded 

 lavers lie near the axis of a synclinal trough, where it is often stated 

 that folding produces tension, not compression. Scrope, for instance, 

 compares folded strata to a bent board, which of course is stretched on 

 its convex side.f Such a comparison is incoiTect. None of the deep- 

 lying rocks can escape compression during their folding : this can 

 happen only to the superficial strata of the anticlinals. That synclinals 

 are actually compressed, and riot stretched, is shown by the growth of 

 subordinate folds often found in their troughs ; by the cleavage of slaty 

 rocks at such points ; and in general by the thickening of (argillaceous) 

 strata at the turns and the thinning at the shanks or tangents of the 

 folds, as was long ago pointed out by Sir James Hall, J and lately 

 confirmed by Heim. § 



Across the stream from the mill is a well-formed synclinal outlier 

 (see Emmons, PI. IV.) capped with Lower Pentamerus and perhaps with 

 some Catskill Shaly limestone. Farther up the gorge, where the stream 

 turns to the rock-strike, there are excellent exposures of Lower Pentam- 

 erus on the eastern (left) bank, and of Catskill Shaly opposite ; the 

 latter dips gently to the west at this point, but following it one third 

 of a mile south, it becomes steeper, and at last is overthrown so as to 

 dip 50° to the east. Above the next band, about at the middle of the 

 west-east course of the stream, on the north bank, a distinct fault may 



* Mechanismus der Gebirgsbildung, 1878, II. 31. 



t Volcanos, pp. 51, note, 289. 



J Edinb. Phil. Trans., VII., 1815, 97. § Loc. cit., II. 48. 



