Mr. Caldcleugh on the Geology of Rio de Janeiro. 71 



some of the beds of gneiss near the sea shore, the crystals of felspar project 

 considerably, and appear to have alone withstood the action of the elements. 

 Some of the masses fallen into the sea appear perfectly fresh until on attempt- 

 ing to stand upon them, the foot sinks into a mass of clay. The felspar is 

 mostly in macled crystals. 



A large proportion of the quartz met with in the neighbourhood of Rio de 

 Janeiro, is of that variety termed slaty and ferruginous. It is by no means 

 unusual to meet large boulders containing beautiful crystals of dark brown 

 mica, of the primitive form. 



The specimen of granite from a drusy vein in gneiss at the Valongo con- 

 tains mica and quartz distinctly crystallized. 



The simple minerals in the gneiss in this neighbourhood are by no means 

 abundant. I met with the following : 

 Hornblende. 



Yellow carbonate of lime, disseminated. 

 Phosphate of lime, amorphous of a deep bottle green. 

 The crystallized variety of the foregoing (Apatite). 

 Garnets, all of some tinge of red, and varying considerably in size. 

 Sulphuret of iron, abundant in the quarry of St. Cristovan. 

 Sulphuret of arsenic. '''>'i?.' 



Chlorite, disseminated and lamellar. 

 With the exception of the garnet, which is more abundant in the gneiss, 

 the other minerals were all taken from granite veins. 



In Europe so large a district of gneiss would have afforded in all probability 

 metalliferous beds or veins of considerable richness ; but in Brazil it is only 

 after leaving this formation, and falling in with the slates of the upper coun- 

 try, that any ores of consequence are to be discovered. 



The following figure represents a case of frequent occurrence, of a bed 

 of gneiss placed unconformably to those of the same rock on which it lies. 



JKwr^jr 



It often happens under these circumstances, that from the decomposition of 

 the lower bed a hollow is formed by the overhanging of the upper ; and I ob- 



