2G4 MR. 0. A. Dr.RRY ON NEl'HELINE-ROCKS IN BRAZIL. 



Aside from the phonolites and foyaitcs discussed above, the other 

 eru])tive rocks, thus far known from Tingua, are of petrographical 

 riitlicr than geological interest. The tendency of certain of the 

 phonolites towards a more highly felspathic type (trachyte) has 

 already been noticed, as ■well as the occurrence of a small dyke of 

 undoubted trachyte (p. 255). The principal geological interest of 

 tliis occurrence arises from the fact that, at another point in the 

 vicinity of Kio de Janeiro (Santa-Cruz branch of the Central Rail- 

 way), the same rock is found in a number of large dykes in the 

 immediate neighbourhood of a plexus of small dykes of i)honolite 

 and various types of basic rocks identical with those of Tingua. All 

 of these occur in a spur from an elongated isolated mountain-mass, 

 the Serra de Mendanha or Madureira, one extremity of which is only 

 five or six miles distant from the Tingua peak ; and they lead to the 

 suspicion that this mass will be found to be very similar to that of 

 Tingua in origin and composition. At the only point thus far 

 examined, a considerable area of a very coarse-grained highlj-- 

 fclspathic rock was found. Such a type would be produced by the 

 disappearance of nepheline from foyaite, and an examination of this 

 mass will probably lead to the discovery of interesting relations 

 between the foyaites and certain more highly-acid types of erup- 

 tives. An interesting occurrence in the Serra de Mendanha is that 

 of a small partially-decomposed dyke of perlite. 



The frequency of small dykes of basic eruptive rocks about the 

 margin of the Tingua mass, and for a distance of several kilometres 

 beyond it. has already been referred to. As yet nothing correspond- 

 ing to the extensive bodies of basalt (leucitite) in the otherwise very 

 similar eruptive mass of Pogos de Caldas has been seen ; but, on 

 the other hand, these rocks are at that place so decomposed and 

 concealed from view, that but for exceptional facilities (extensive 

 railway-cuttings in the deeper portions of the mass), such as do not 

 exist at Tingua, they would never have been discovered. All things 

 considered, the occurrence in the upper part of the Eio do Ouro 

 valley of large masses of some sort of basic rock is extremely 

 probable. The dykes seen vary in thickness from 10 cm. to 1 m., 

 and, with the exception of three found in the tufaceous phonolite, 

 are in gneiss. 



The rocks of these dykes vary considerably in mineral compo- 

 sition and aspect under the microscope, no two being exactly alike ; 

 but all present certain characters in common, which led Prof, 

 liosenbu^ich (to whom a considerable collection was submitted) to 

 lump them all together as " a peculiar group of augitite " *. This 

 view, based upon petrographic considerations, is in perfect accord with 

 their geological occurrence, since not only at Tingua, but at three 

 other localities, the various phases occur together, and in the imme- 

 diate vicinity of eruptive centres characterized by nepheline rocks. 

 Moreover, at one of these localities (near the Serra de Mendanha) 

 basic aggregates, closely resembling certain rocks of this group, 

 have been observed in phonolite. The fundamental type presents a 

 colourless glass-basis with microliths of augite, generally accom- 

 * ' Mikr. Phys. Gesteine,' 2nd ed. p. 821. 



