OCCLUSION OF IGNEOUS ROCK 425 



may be reckoned to have consisted of bronzite 26.1, and diallage 

 or diopside 73.9 parts. 



In the Staten Island rocks the composition is aUied to that 

 of combination of oHvine and bronzite with diallage. The pre- 

 dominance of amphibole rich in lime and magnesia, with traces 

 of pyroxene, olivine and bronzite, suggests derivation directly 

 from a basic hornblende schist, derivative in its turn from an 

 ancient diallagite-, bronzite- or enstatite-rock, passing at one 

 point at least into peridotite. The prevailing secondary altera- 

 tion, e. g., at Pavilion Hill, of diopside into tremolite, indicates 

 an increase in volume of 5.6 per cent., or into talc, of 30.1 per 

 cent., and of olivine into serpentine, etc., 12 to 37 per cent.^ 

 In this rock the superior proportion of serpentine may give a 

 clue to the original amount of olivine. From the ratio of ser- 

 pentine to diallage at Hoboken, the proportion of the former at 

 Staten Island would be 13.76 per cent. The excess, 24.72 per 

 cent., would contain 10.44 P^^' cent, of magnesia, mainly due to 

 olivine. From the percentage of magnesia included in brucite, 

 in olivine-serpentine, and in derivatives from diallage, the fol- 

 lowing constitution of the original peridotite may be inferred : 

 bronzite 21.3, olivine 36.6, and diallage 42.1 parts. These 

 determinations of mineralogical constitution, it will be remem- 

 bered, apply merely to the specimens represented in Goodell's 

 analyses. At Staten Island, the amount of pyroxenite prob- 

 ably far exceeded that of peridotite. 



Phases of Alteration. — The serpentinoid mass on the west 

 bank of the Hudson River and harbor appears therefore 

 closely allied to that on Manhattan Island, at West 59th 

 Street, and of a like basic type — an intrusive sheet or sill of 

 greater dimensions, but of the same constitution, sheared struc- 

 ture and mode of alteration. The metamorphism of these old, 

 more or less basic occlusions has presented two phases. 



Where the original intrusion was feldspathic, probably a 

 bronzite- or enstatite-gabbro (like that of Chester County, 

 Pennsylvania ^), the constitution favored the genesis of diorite, 



^Van Hise, op. cit., 274-275, 388. 



2 F. D. Chester, Sec. Geol. Surv. Penn., Ann. Rep., 1887, 100-102. 



