598 THE JOURNAL OF GEOLOGY. 



with the predominant foyaite, phonolite and basaltic rocks, which 

 have since been named monchiquites by Prof. Rosenbusch, 

 occurred. These two last types, found only in loose blocks or in 

 small dykes in gneiss that was clearly older than the foyaite, 

 gave no idea of their relations to the latter rock except that at 

 one point a small dyke of phonolite containing polyhedral inclu- 

 sions of foyaite, like raisins in a pudding, was observed cutting 

 foyaite of the same type as the inclusions. An examination of 

 a series of railroad cuttings between the peak and the city 

 showed a plexus of phonolite and monchiquite dykes together 

 with a peculiar feldspathic rock of syenitic aspect, which, as 

 they did not extend to the city, were suggestive of a possible 

 genetic connection with the eruptive center of Tingua, or of some 

 other similar center in the vicinity. 



The occurrence of phonolites, hitherto only known on Bra- 

 zilian soil on the volcanic island of Fernando de Noronha, sug- 

 gested a search for phonolitic centers of eruption. About this 

 time a chance collection made by a naval officer from the island 

 of Cabo Frio, 60 miles from Rio, came to hand. As it con- 

 tained specimens of both phonolite and foyaite, an excursion 

 was resolved upon, guided by the thought that a rocky island 

 on an open coast should give good exposures and thus perhaps 

 prove a better point than Tingua for the stud)- of the problems 

 presented in this mountain. The island, from two to three 

 miles long and from one-fourth to one-half mile wide, was found 

 to give an almost continuous rock exposure about its entire 

 margin. About four-fifths of the island is composed of coarse 

 grained sodalite-bearing foyaite somewhat different from the 

 Tingua type, and like it cut by numerous dykes of phonolite. 

 The remainder consists of augite-syenite of two types, except a 

 small point which is distinctly tuffaceous and cut by innumerable 

 small dykes of a basaltic character. In one place dyke-like 

 masses and large boulder-like inclusions of a pyroxene-plagio- 

 clase rock of a gabbro type occur. The coast of the mainland, 

 distant half a mile more or less from the island, is entirely free 

 from rocks of a syenitic character, and is composed of gneiss cut 



