NEPHELINE-EOCKS IN BRAZIL. 



471 



it are somewhat difficult to find. It is seen, however, to be most 

 pronounced near the surface and in the neighbourhood of fissures, 

 and is undoubtedly due to weathering or to infiltrating waters. 

 This phonolite differs further from all other varieties seen in the 

 region, except that of the Prata bridge, in being fit for a building- 

 stone on account of its freedom from the fine joints and splintery 

 fractures that generally characterize the Caldas phonolites. About 

 the village of Pocos the phonolite is generally distinctly granular, 

 and might perhaps be regarded as a fine-grained foyaite. The 

 hot sulphur springs (temp. 45° C.) that give name and importance 

 to the place issue from the midst of the phonolite. The hill close 

 to the village (1600 metres high, the highest of the semicircular 

 ridge) was examined for a distance of over a mile, and found to con- 

 sist exclusively of phonolite. This rock also characterizes the road 

 from Pocos to the Rio-Pardo margin of the plateau, occurring also 

 with foyaite along the descent, but, apparently, not extending 

 beyond the river. 



The Rio das Antas, the principal stream of the plateau, just 

 before it breaks through the ridge to descend to the Rio Pardo, 

 traverses for several hundred metres a considerable patch of 

 quartzite. In places this shows the flaggy structure and other cha- 

 racteristics that identify it with the itacolumites so abundant in the 

 province of Minas. The geological age of the series to which it 

 belongs has not been determined for Brazil, but it is certainly very 

 old. Hocks similar to it and to its associates are called Huronian 

 by many North -American geologists. The strike is N. 20° W. # ; 

 dip 20° S.W. A small exposure of phonolite occurs close to the 

 quartzite, and two quarries in foyaite have been opened in the imme- 

 diate vicinity, but the relations of these rocks to the quartzite could 

 not be seen. A little above a cascade formed by the quartzite there 

 is another, in which the rock, at first sight, appears to be similar ; 

 but on closer inspection it is seen to be a greenish tuff enclosing 

 fragments of eruptive rocks. This continues for a considerable dis- 

 tance up stream, then isolated masses of quartzite begin to appear 

 associated with it, though in no case could an actual contact be 

 discovered, nor could pebbles of quartzite be detected in the tuff, 

 though grains of quartz and fragments of gneiss are not uncommon. 

 Apparently, however, the tuff forms a layer on an irregular surface 

 of quartzite, points of which occasionally appear through it. The 

 tuff is traversed by joints running N. 70° E., and dipping 70° JN"., 

 which in places produce a sort o£ flaggy structure. 



The Rio Pardo appears to form the northern limit of this eruptive 

 group as the Bio de Prata forms the southern. Going northward 

 from the Rio Pardo only gneiss, mica-schist, and granite were met 

 with between that river and the Rio Grande. North of the latter 



* It may here be remarked that I found a north-westerly strike predomi- 

 nant between Caldas and the Serra clo Oanastra, as well as in that range, whereas 

 in Eastern Minas and along the Serra do Espinha^o the strike is almost univer- 

 sally north-westerly. It is for this reason that I have ventured to suggest (by 

 the dotted line of the map, fig. 1) the derivation of the Serra do Oanastra as 

 a branch of the Mantiqueira range. 



