PHYSIOGRAPHIC RELATIONS OF SERPENTINE 585 



dotite, we find in the process of serpen tinization and the consequent 

 inevitable expansion the requisite innate power of growth and 

 topographic rejuvenation. This alteration, of which hydration is 

 now the most important phase, and which is simplest, most direct, 

 and most important for the mineral olivine, involves in every case 

 a notable diminution of density and increase of volume, the volu- 

 metric gain, according to Van Hise,"" ranging, with different species 

 and varying conditions, approximately from 15 to 40 per cent. It 

 is, of course, to this notable expansion that we owe the tightness 

 of the joints and the internal slickensides and slip-fiber veins so 

 characteristic of massive serpentine. 



Under the conditions surrounding an approximately vertical 

 plug or stock, at a great depth in the earth, the expansion due to 

 serpentinization cannot- be, to any large extent, isometric (cubic), 

 but must take place chiefly or wholly upward, that is, in the direc- 

 tion to give it the maximum topographic value. This means that 

 for the maximum ratio of expansion, 40 per cent, and ignoring con- 

 temporaneous erosion, the surface exposure of the serpentine stock 

 would be elevated nearly 40 feet for every 100 feet in depth of 

 peridotite altered to serpentine; or one-half as much for an assumed 

 mean expansion of 20 per cent. These relief or topographic values 

 may, however, be exceeded in the cases, probably numerous, where 

 the diameter of the stock increases downward, the sepentine then 

 reaching the surface by accelerated flow through a more or less 

 contracted vent. 



If we assume, with Van Hise,^ that serpentinization is limited to 

 the zone of katamorphism, the estimated depths of which range 

 from 30,000 to 40,000 feet, the total vertical expansion in this zone, 

 even for the minimum ratio, would still account many times over 

 for the relief of the pseudo-monadnock of Staten Island Heights — • 

 a monadnock developed, in the main at least, subsequently to the 

 peneplanation and in a manner suggesting comparison, rather, with 

 the growth of the spine of Mont Pele. 



We have here, apparently, a new physiographic type, a variety 

 of auto-relief not heretofore clearly recognized; and comparison 

 with the spine or obelisk (pelelith) of Mont Pele is inevitable The 

 pelelith, we may suppose, resulted from the extrusion (or protrusion) 



^ U.S. Geo!. Survey, Monograph 47. ^Ibid. 



