1892.] 



NEW YORK ACADEMY OF SCIENCES. 



61 



with a much more prismatic development of the elaeolite in the 

 ground mass. 



An analysis yielded the results in column I. Column II is of 

 the elaeolite porph}^'}^ from Magnet Cove (J. F. Williams, Igneous 

 Rocks of Arkansas, p. 261). 



Loss 

 SiO., 

 TiO„ 



Al-A 



Fe,03 



FeO 



CaO 



MgO 



K„0 



98.92 



99.59 



The rock is thus more basic than the syenite, and illustrates a 

 change to a more basic composition as the south end of the dike is 

 approached. It is very basic also when considered as an analogue 

 of phonolite, and is nearer the theralites. With some variations it 

 has other close points of correspondence with the analysis quoted 

 by its side. 



The sections of the New Jersey phonolitic or tinguaitic specimens 

 have been compared with a set which were made from a suite of the 

 corresponding Brazilian varieties which are in the rock collections of 

 Columbia College. They were presented some years ago by Mr. 

 D(!rbv. The resemblance is most striking and indeed so close that 

 the slides might readily be confounded. The New Jersey examples 

 show no tendency to cleave in thin plates like ordinary phonolites 

 and like some of the Brazilian examples, but break in angular frag- 

 ments ; hence the name elaeolite-porphyry seemed appropriate and 

 was employed above. The correlation of many types in the two 

 exposures is remarkable. 



Still further south, at a point half-way between the last and the 

 southern extremity of the dike, a prospect hole has been blasted by 

 some one in the top of a conical hillock. It has opened up a cavity 

 some few feet deep, and brought out a quantity of perfectly fresh 

 material. The rock is rather finely crystalline, dark in color, and 

 very tough. It looks like a fine mica diorite, and has a great pro- 

 portion of dark silicates. The sections exhibit elaeolite, orthoclase, 

 aegirine, biotite, titanite, many minute zircons and apatites. Ex- 

 cept the minute inclusions, all the components are allotriomor- 

 phic. The aegirine and biotite especialh^ appear in shreds and small 

 irregular masses. No cancrinite is present, and from this fact, as 

 the rock is perfectly fresh, the conclusion is unavoidable that the 

 cancrinite to the north is secondary and derived from the elaeolite. 

 Careful search was made for plagioclase in the slides, yet none 



