J. V. Lewis — Palisade Diabase of New Jersey. 155 



Art. XIX. — The Palisade Diabase of New Jersey; by J. 

 Yolney Lewis.* 



The intrusive trap that forms the Palisades of the Hudson 

 extends, with outcrops several hundred feet thick, from west 

 of Haverstraw, JN". Y., southward to Staten Island and. some- 

 what intermittently, westward across New Jersey to the Del- 

 aware River, an aggregate length of about 100 miles. \ It is 

 everywhere a medium- to fine-grained dark gray heavy rock, 

 with dense aphanitic contact facies. 



The typical coarser rock contains, in the order of abundance, 

 augite, plagioclase feldspars, quartz, orthoclase ) magnetite, and 

 apatite. The tirst two occur in ophitic to equant granular tex- 

 ture, and the next two in graphic intergrowths which some- 

 time constitute one-third of the rock ; in the contact facies 

 this micropegmatite disappears and scattering crystals of olivine 

 occur. 



A highly olivinic ledge, 10 to 20 feet thick and about 50 

 feet from the base of the sill, is exposed in the outcrops north- 

 ward from Jersey City for about twenty miles. The olivine 

 crystals, which constitute 15 to 20 per cent of this rock, occur 

 as poikilitic inclusions in the augite and feldspar. 



Chemically the diabase ranges from less than 50 to more 

 than 60 per cent of silica, with corresponding variation in 

 alumina, ferric oxide, and the alkalis, while ferrous iron, lime, 

 and magnesia vary inversely. The augite is rich in these latter 

 constituents and poor in alumina, giving a great preponder- 

 ance of the hypersthene and diopside molecules. The feld- 

 spars range from orthoclase and albite to basic labradorite. 

 Doubtless there is some anorthoclase since all feldspar analyses 

 show potash. 



While there is considerable range in the proportions of the 

 minerals, augite usually comprises about 50 per cent of the 

 rock, the feldspars about 40 per cent, quartz 5 per cent, and 

 the ores 5 per cent, constituting a quartz-diabase, with normal 

 and olivine-diabase facies. Basic concentration at the contacts, 

 followed by differentiation by gravity during crystallization 

 of the body of the sill, especially by the settling of olivine and 

 the ores and the rising of the lighter feldspars in the earlier 

 and more liquid stages of the magma, are hypotheses that seem 

 to account for the facies observed and their present relations. 



Microscopic cho.racters. — In thin sections the texture of the 

 rock is usually diabasic, or ophitic ; that is, the augite fills the 



* Eead before the New York Academy of Sciences April 6, 1908. Published 

 by permission of the State Geologist of New Jersey. 



\ J. Volney Lewis, Structure and Correlation of the Newark Trap Eocks of 

 New Jersey, Bull. Geol. Soc. of America, vol. xviii, pp. 195-210; also 

 Origin and Relations of the Newark Eocks, Ann. Eept. State Geologist of 

 N. J. for 1906, pp. 97-129. 



