256 UlR. 0. A. DERBY ON NEPIIELINE-ROCKS IW BRAZIL. 



that it was impossible to obtain a specimen suitable for microscopic 

 examination. Indeed, it was extremely difficult to detach any sort 

 of specimen, as the rock is smoothed and rounded by the stream like 

 a mass of stiff clay, and behaves like it under the hammer. A 

 ])olished face of some of the harder pieces detached sliows a typical 

 l)lionolitic structure, with large and well-formed phenocrysts of 

 felspar, completely kaolinized, and quadratic sections of a milky- 

 white zeolite (?), which probably represents nepheliue. Another 

 specimen is a true tuff, with large and small rounded pebble-like 

 fj-agments, in part granular, in part compact. On the water-worn 

 walls of the canon the tufaceous character of a large part of the mass 

 is very api)arent, fragments, up to the size of a man's head or larger, 

 standing out very distinctly from the general mass. Some of these 

 are coarsely granular, and appear to be foyaite ; others are compact 

 like phonolite. Iso line of demarcation between the part with in- 

 clusions (tuff) and the ordinarj' phonolitic type could be observed, 

 since, if any such existed, it has been obliterated by the equal decay 

 of the whole mass. The essential fact, however, is clear, that a 

 true effusive rock here occurs, represented in part by fragmental 

 eruptions, in part probably by a phonolitic lava-llow, although it is 

 ]>ossible that the phonolite specimens may have come from a dyke 

 traversing the mass of tuff. A complete analogy is thus established 

 with the Pocos de Caldas locality, where foyaite, phonolite, and 

 fragmental eruptives occur so intimateh' associated that a truly 

 volcanic origin, in the most restricted significance of the term, may 

 safely be predicated for the whole. 



This conclusion of a volcanic origin for the eruptive mass of 

 Tingua, including its most characteristic rock-types — the foyaites, 

 affords a ready explanation for a number of peculiarities of this 

 rock, which are difficult to account for on the generally accepted 

 view that it is a deep-seated rock {Tiefengestein), in the same sense 

 as are, for example, the granites and ordinary syenites. These 

 peculiarities are, as regards the Tingua mass, the mode of occur- 

 rence, the irregular {Schlkren) structure, and trachytic (phonolitic 

 or porphyritic) habit. The last two points have been noted in other 

 foyaite localities * ; the first, so far as I am aware, has not received 

 any particular attention. 



So far as can be concluded from the examination hitherto made, 

 the foyaite of the Serra de Tingua nowhere presents the character- 

 istics of a dyke or boss. A complete circuit of the mountain has 

 been made within a distance of 1-2 kilometres of the area in which 

 foyaite is the predominant surface-rock, and often within it, with- 

 out meeting a single mass of the I'ock in situ. The cuttings of the 

 pil)e-, tram-, and ordinary road-lines, amounting to some thirty 

 kilometres or more of rock in situ, are uniformly in gneiss cut by in- 

 numerable dykes of granite, diabase, phonolite, and basalt, but with- 

 out a single dyke' of foyaite, although in scores of cuttings the 

 latter rock is present in loose rounded blocks, resting on the 



* See Eosenbuscb, ' Mikr. Phjs. Gesteiue,' p. 92. 



