NEPHELENE-EOCKS EST BRAZIE. 



465 



metre, mingled together in the greatest confusion, and loosely- 

 cemented by a paste of pebbles and minute ronnded grains of 

 the same nature as. the boulders. With the exception of a few 

 masses which are fragments of a preexisting and more firmly com- 

 pacted conglomerate of the same aspect as that of which they now 

 form a part, all the boulders and pebbles seen are of the same 

 character and, to my eye at least, undistinguishable either macro- 

 scopically or microscopically (except by a slight difference in colour, 

 a light shade of red or a leaden colour being predominant) from the 

 red rock of the adjacent cuttings. At one point a small mass of the 

 red rock, decomposed, but evident!)' in situ* rests upon the con- 

 glomerate, which is also cut by a small dyke which, in its decom- 

 posed state, also resembles the red rock. A relation is thus estab- 

 lished between this large and almost detached mass of conglomerate 

 and the smaller patches, which are clearly included in the red rock, 

 and the latter is thus seen to pass on the one hand into a fragmental 

 rock, and on the other into a compact phonolite *. 



On the same spur, between the tunnel and Pinhalzinho, occurs 

 the largest exposure yet known in the region of foyaite, which 

 evidently forms a considerable portion of the mass of the spur, and 

 appears in connexion with the red rock throughout a distance of 

 about 2 kilometres. The rock is in general of rather coarse grain, 

 but of even texture, and weathers into large rounded boulders, 

 which, if the massive rock was not seen in the cuttings, might be 

 taken for erratics. In one place, at the side of a small ravine, the 

 texture is porphyritic, with large and small polygonal patches of 

 coarse-grained whitish rock and large and perfect felspar-crystals 

 scattered through a bluish finely granular ground-mass, in which, 

 however, the granitic texture is still apparent. At the tunnel the 

 relations of the foyaite to the red rock are very well exposed. 

 High up on the side of the peak above the tunnel a considerable 

 mass of foyaite is seen close alongside of a considerable exposure of 

 the red rock. The tunnel is excavated in a large irregular dyke- 

 like mass of foyaite that cuts the red rock, and is most probably 

 continuous with that of the top of the peak some 300 metres above 

 it. This mass of foyaite is separated by an intervening mass of red 

 rock from another about 100 metres further up the ravine, in which 

 a quarry has been opened. The annexed sketches (fig. 4) of the two 

 openings of the tunnel and of parts of its sides near the upper end 

 show the relations of the two rocks. At the upper end, the lower 

 part of the arch is of the red rock, rising highest on the left or 

 upper hill-side ; the upper part is of foyaite. A road cut on the 

 right side, on a level with the floor of the tunnel, shows the foyaite 

 cutting out the small patch of red rock of the right-hand side of the 

 mouth, but giving way to it again a little further round the hill. 

 This appearance can onfy be explained by regarding the fo} T aite as 

 an irregular dyke-like mass, some 10 metres or more thick, cutting 



* Judging from the reference in Eosenbusch's ' Mikroskopische Physio- 

 graphic,' vol. ii. p. 299, a comparison might be made between this rock and 

 that from Tenerifi'e denominated eutaxite by Pritsch and Eeiss. 



