SYENITE AND PORPHYRY OF NEW JERSEY 575 



early Tertiary and is probably much older) at the junction of the 

 Shawangunk conglomerate and the Martinsburg shale, is critical, 

 and, taken in conjunction with the fact that long, narrow intrusions 

 of nephelite syenite and bostonite lie parallel to the bedding of the 

 Martinsburg shale farther to the east, inchnes us to the opinion 

 that the Beemerville mass is a lenticular sill, or a flat laccolith. 



The nephelite syenite is a somewhat basic foyaite, of the "foy- 

 aite range" as recently defined in the classification of the nephelite 

 syenite family proposed by Shand.^ It is very variable along the mass 

 and, although the bulk of the exposure is a fairly constant foyaite 

 of the Magnet Cove, Arkansas, or the Umptek type, it grades 

 locally into other facies, which are often more basic than the main 

 mass. Ditroitic and ijoHtic modifications may be collected, and 

 especially along the eastern border it becomes foliated, or lujavritic, 

 in character. Near the center of the exposure, small, local facies 

 with abundant titanite may be found. These variations have been 

 admirably described by Emerson and by Kemp. Kemp's descrip- 

 tion may be quoted to illustrate this point: 



The dike varies considerably along its course. The typical elaeoUte- 

 syenite forms the northern third and the southern extremity, but between these 

 points its character changes. Near the southern part of the middle third 

 elaeolite-porphyry appears, and forms a most beautiful example of this rock. 

 It may come from dikes, as no actual exposures are available. Further south 

 a basic holocrystalhne rock comes in which is exposed in place; and, as subse- 

 quently shown, contains less silica and more biotite than the typical syenite. 

 But on the extreme south where the highway crosses the dike, the rock is much 

 like that on the north. It is, however, greatly decomposed, and fresh, firm, 

 pieces are hard to find.^ 



The variation, even of what appears to be the predominant rock, is 

 well shown by comparing the analyses by Eakins and Aurousseau 

 (Table I). The Martinsburg shale, along the eastern contact, 

 has been metamorphosed to a homstone, the aureole being narrow. 

 To the very complete petrographic description of the normal 

 rock, given by Iddings, we have little to add. One sHght correction 

 is necessary. The mineral identified as sodalite belongs to the 

 hauynite-nosehte series, as is indicated by the analysis here given. 



' S. J. Shand, Trans. Geol. Soc. South Africa, XXIV (1921), p. 117. 

 " J. F. Kemp, Trans. N.Y. Acad. Sci., XI (1892), p. 64. 



