600 THE JOURNAL OF GEOLOGY. 



decomposed crystals, at first taken for analcime, as well as 

 polyhedral inclusions similar to those of the phonolite of 

 Tingua were obtained. To complete the felicity of the excursion 

 a cutting at the foot of the mountain showed the eruptive rocks 

 to be in part, at least, contemporaneous with Carboniferous strata. 



With the data here obtained a paper was prepared and pre- 

 sented to the Geological Society of London (Quart. Jour. 43, 

 1887) announcing the discovery and general distribution of 

 nepheline and leucite rocks in Brazil, and the general conclusion 

 that the Pocos de Caldas eruptive center is volcanic in the most 

 restricted sense of the term, that it is of Carboniferous age, and 

 that here foyaite and phonolite occur as different phases of the 

 same magma. 1 



The attack on Tingua was now renewed with the expectation 

 that a diligent search would reveal something analogous to the 

 Caldas region. A trip to the top of the peak showed little of 

 interest beyond a dyke of phonolite cutting foyaite at the very 

 summit. An examination of the margins, well shown by the 

 cuttings of an extensive series of railroad and pipe lines (for the 

 water supply of Rio) at the front, a river valley at the back and 

 roads over the ridge at both ends of the peak, showed that the 

 foyaite is limited to the massif and nowhere presents unequivo- 

 cally the character of dykes. Two cuttings, one a tunnel, through 

 a spur covered with foyaite boulders as if from the outcropping 

 of a dyke, is conclusive on this point, as only gneiss was found 

 in situ. The eruptive rocks are therefore placed like a plaster on 

 the top and slopes of a gneiss ridge in a manner exceedingly sug- 

 gestive of volcanic conditions. By forcing a way through the 

 dense forest into the crater-like valley of a stream coming from 

 the very heart of the mountain, the long-sought-for evidence of 

 fragmental eruptives and of extensive masses of phonolite in 



1 Subsequent explorations of the Caldas center proves it to be one of the grandest 

 volcanic masses of nepheline rocks known, measuring from fifteen to twenty miles in 

 diameter. Contrary to the first impression the foyaite masses are comparatively 

 insignificant, and the massif is composed essentially of phonolite and tuff with possibly 

 a large proportion of basic leucite rock. A large and important mass of augite-syenite 

 appears to form part of the same volcanic massif. 



