66 TRANSACTIONS OF THE [fEB. 1, 



resembles a secondary filling of a crack, as the plates lie across the 

 long direction of the flow lines. A short distance from the contact, 

 however, the dike becomes the coarsely crystalline syenite and extra 

 rich elaeolitic portions could only be extremely local. 



I searched carefully and in man\^ places for endialyte or eucolite, 

 but none appeared. Professor Rosenbusch notes small crystals of 

 a questionable mineral in a slide of a finel}' crystalline variety sent 

 him, as perhaps one of these. One or the other may be discovered 

 in the future. In the writer's opinion it would not be worth while 

 to make separate varieties of the rocks with aegirine and of those 

 with liiotite, as there is hardly a pure specimen of either. Experi- 

 ence in this occurrence thus corroborates the general conclusions 

 laid down by Rosenbusch about the subdivisions of the elaeolite- 

 syenites. Small hornblende crystals regarded as arfvedsonite are 

 noted by Emerson, but they have escaped my attention if present 

 in my slides. 



At the middle point of 4 C on the map the character of the dike 

 changes, as is indicated by the float fragments, for no actual expo- 

 sures occur. Pofphyritic facies appear, and an excellent elaeolite- 

 porphyry was found. By an odd coincidence the same kind of rock 

 was discovered at almost the same time by the late Dr. J. Francis 

 Williams, with whom the writer was in active correspondence, in 

 Saline Co., Arkansas, making the first American records of this 

 rare rock species simultaneous in two widely separated regions 

 (see Igneous Rocks of Ark., p. 149). The Beemerville porphyry 

 is dark greenish in color, and has great hexagonal phenocrysts of 

 elaeolite up to an inch in cross section. The slides show a ground 

 mass perfectly typical of the dikes recently called tinguaite by 

 Rosenbusch, from the Brazilian occurrences. They furnish a struc- 

 ture among the elaeolite dike rocks closely analogous to the phono- 

 lites of the effusives, and the dikes were indeed called phonolites by 

 Derby, their original discoverer.^ The ground mass consists essen- 

 tially of elaeolite surcharged with microscopic aegirine needles. 

 Orthoclase is present and many crystals of a very peculiar pyroxene. 

 This is light yellow in color, with a pleochroic change along axial h 

 to a pinkish shade. It is idiomorphic, and has an extinction that 

 may reach 44°. Professor Rosenbusch estimates the angle of the 

 optic axes at about 55°. An optic axis emerges not greatly inclined 

 to the basal section. 



Closely associated with the pyroxene is a reddish-brown nearly 

 isotropic mineral, of high index, that is much Mke perofskite. A few 

 shreds of biotite are also seen. 



Another porphyritic rock occurs along this portion of the dike, 

 which lacks the large phenocrysts of elaeolite. It has, however, 

 others of feld.spar, and in the slide shows the same tinguaitic base 



1 0. A. Derby, On Nepheline Rocks in Brazil with Special Reference to the 

 Association of Phonolite and Foyaite, Q. J. G. S., August, 1887. 



