104 



Lyman.] -^'^-'* [May 18, 



observed facts, it lias become a heresy to maintain a thickness different 

 from the still more purely conjectural one of between 3000 and 5000 feet. 

 Then naturally followed conjectures to account with that moderate 

 thickness for so great a geographical breadth in spite of the known dips. 

 These conjectures have been ingenious and elaborately argued and zeal- 

 ously adhered to, but have one by one been disproved or found to be at 

 best only imperfectly supported by observation. It was thought that the 

 dips might be merely apparent or due to false bedding, deposition on a 

 sloping surface, but the thin-leaved, slialy character of some of the beds 

 and the position of the pebbles, ripple marks and fossil footprints have 

 shown the impossibility of that supposition. It was further conjectured 

 that a series of great parallel longitudinal faults with downthrow con- 

 stantly in one direction might diminish the thickness to the required ex- 

 tent, but their main support was the very insufiScieut one that recurring 

 hard beds or parallel hills had a similar red color. A careful consideration 

 of the very much curved strike of the beds in some parts of Pennsylvania 

 and New Jersey shows that no series of parallel great faults would help 

 the matter. Besides, although faults of a few feet or yards are numerous, 

 their direction is not generally longitudinal nor the downthrow uniformly 

 in one direction, and but one great fault has yet been proved to exist, 

 and that only in Pennsylvania and New Jersey, and by no means gener- 

 ally longitudinal. 



Conjectures in regard to the trap, supposed to be so important in 

 "indurating" and darkening the New Red, have been, if possible, even 

 more wild and needless. The impression seems generally to have been 

 very strong that every mass of trap must be a dike, and that if it was 

 undeniably interbedded conformably witli the shales, it must necessarily 

 be a dike that closely followed the bedding intrusively, no matter how 

 many miles, no matter how soft the shales, no matter how gentle the dip. 

 Sometimes it was preposterously suggested that the trap had occasioned 

 the dip of the shales, both near to it and far away. But, in general, as 

 much advantage as possible was taken of the dip, and the trap supposed 

 to be intruded after the dip had been fully acquired, quite dissociating 

 certain sheets of trap from the age of the New Red sedimentary beds with 

 which all the trap is otherwise so closely connected, and not considering 

 that the dip is even now probably still in process of gradual acquirement, 

 or by occasional small fits and starts (witness the earthquake that was 

 felt only the other day between Lambertville and Flemington, N. .!., near 

 the line of the great fault there, and corroborative of the existence of the 

 fault at the place pointed out in a former communication, Proc. Amer. 

 Philos. Soc, Vol. xxxi, p. 314). Yet, as the dip alone is so gentle that a 

 dike following it must have come from many miles' distance to have 

 originated at a depth great enough to be melted, and could hardly be sup- 

 posed to refrain for so long a space from sometimes breaking across the 

 soft shales by a short cut to the surface, it was imagined that the dike 

 must be nearly vertical at a short distance below the outcrop. Then as 



