472 



ME. 0. A. DEE BT ON 



the quartzite (itacolumite) and metamorphic-schist series (repre- 

 sented in the Caldas region by the quartzite of the Rio das Antas 

 and Cascata) appears in the well-defined mountain -range known as 

 the Serra do Canastra, which forms the western wall of the Sao 

 Francisco basin and the watershed between that river and the upper 

 Parana. The sandstones and bedded melaphyres of the upper part 

 of the Sao Paulo plateau abut against the western side of this range, 

 their eastern margin cutting across the rivers Pardo and Grande, 

 some distance to the westward of the Caldas mountains. Gneiss 

 is said to occur on the mountainous plateau near the city of Caldas, 

 and fine and large crystals of zircon come from the same locality, 

 leading to the suspicion that zircon-syenite or a zirconiferous variety 

 of foyaite may occur in that vicinity. The limited time at my dis- 

 posal did not permit me to follow up this interesting suggestion *. 



Pending the detailed microscopic examination of the Caldas rocks 

 which Prof. Rosenbusch has kindly undertaken, it does not seem 

 desirable to attempt to go very far in drawing conclusions from the 

 observations recorded in this interesting region. It is hoped, how- 

 ever, that this imperfect exposition may prove as convincing to the 

 minds of others as the phenomena observed in the field were to my 

 own mind on the following points : — 



1st. The substantial identity as regards mode of occurrence and 

 geological age of the Caldas phonolites and foyaites. 



2nd. The connexion of the latter through the phonolites with a 

 typical volcanic series containing both deep-seated and aerial types 

 of deposits. 



3rd. The equal, if not greater, antiquity of the leucite-rocks as 

 compared with the nepheline-rocks, whether fclsitic, as phonolite, or 

 granitic, as foyaite. 



4th. The probable Palaeozoic age of the whole eruptive series. 



The interruption in the section of the sedimentary series ne*ar the 

 foot of the Caldas mountains renders the last conclusion less positive 

 than could be desired. The conglomerate at the base of the sand- 

 stone at the foot of the mountain proves that the eruptions com- 

 menced before or during the deposition of the sandstone, while the 

 dykes cutting the latter show that they continued after the deposi- 

 tion. It is certain that at the time when the present sedimentary 

 plateau of Sao Paulo was occupied by a late Palaeozoic or early 

 Mesozoic sea, the Caldas mountains, with an elevation of at least 200 

 metres, rose either at the margin, or not more than 10 miles distant 

 from the margin of that sea, and this, in accordance with the law of 

 the relation of volcanoes to the coast-line, may be taken as an argu- 

 ment in favour of the great age of the eruptions, since no late 

 Mesozoic or Tertiary marine deposits are known nearer than the 

 present Atlantic coast between Rio de Janeiro and Bahia, or the 

 Pampa region of the Paraguay basin. Freshwater Tertiary deposits 

 are known about the city of Sao Paulo and between the Serras do 



* Zircon has not been recognized in any of the Brazilian foyaites thus far 

 examined. Sphene is a characteristic and abundant microscopic accessory in 

 that of Oabo Frio, Tingua, and Itatiaia, but has not been observed in that of 

 Caldas, which also differs in other respects from that of the other localities. 



