PHYSIOGRAPHIC RELATIONS OF SERPENTINE 591 



(Lafayette) scantily preserved on a bench or terrace above the steep 

 southeast slope of the serpentine ridge, at elevations of 180 to 220 

 feet above the sea. Since this gravel is identical in character with 

 the Lafayette (Beacon Hill) of New Jersey, and the elevation is 

 that normal for the Lafayette in the latitude of Staten Island, this 

 correlation appears inevitable; although it involves the conclusioii 

 that the elevation of the marginal part of the serpentine, relatively 

 to the Cretaceous peneplain, has remained unchanged since Pliocene 

 time; with the further suggestion that the central and higher part 

 of the serpentine (180 to 360+ feet) has experienced a differential 

 elevation of nearly 200 feet in post-Pliocene time. An alternative 

 but not very probable view is that the serpentine was not com- 

 pletely base-leveled in Pliocene time. That the central part of the 

 stock should rise most rapidly or should continue to rise after the 

 elevation of the periphery has virtually ceased is not difhcult to 

 understand, especially if the stock becomes larger downward, since 

 in that case serpen tinization of the central part may be conceived as 

 lagging behind the serpentinization of the peripheral part. 



As the surface of the ground is lowered by erosion, the lower 

 limit of the zone of katamorphism and of serpentinization must be 

 correspondingly depressed; although it is probable that the latter 

 effect tends to lag behind the former. Downward extension of ser- 

 pentinization means a 1 5 to 40 per cent or greater upward extension 

 of the summit of the serpentine, the topographic relief of the serpen- 

 tine being, thus, constantly renewed and increased. It is probable, 

 however, that when erosion was sufficiently long continued with 

 reference to a definite base -level, as during the Jurassic and early 

 Cretaceous subaerial peneplanation and marine planation of the 

 site of Staten Island, the process of serpentinization, overtaking the 

 slowly receding and finally stationary lower limit of the zone of 

 katamorphism, was, for the time being, virtually exhausted. The 

 upward growth of the serpentine then ceased; and it was, at least 

 approximately, base levelled with the surrounding formations, and, 

 in due course, covered by the conformable series of Upper Creta- 

 ceous and Eocene sediments. 



The marked elevation and consequent extensive erosion of the 

 land in early Miocene, substantially repeated in early Pliocene time, 

 probably depressed the lower limit of the zone of katamorphism 



