426 Rio de Janeiro : past n. 



varieties of gneiss regularly foliated, and alternating 

 with each other in so-called strata. The stratification 

 and foliation of the ordinary gneisses, and the folia- 

 tion or { grain ' of the gneiss-granite, are parallel to 

 each other, and generally strike within a point of XE. 

 and SW. dipping at a high angle (between 50° and 60°) 

 generally to SE. : so that here again we meet with the 

 strike so prevalent over the more northern parts of this 

 continent. The mountains of gneiss-granite are to a 

 remarkable degree abruptly conical, which seems caused 

 by the rock tending to exfoliate in thick, conically 

 concentric layers : these peaks resemble in shape those 

 of phonolite and other injected rocks on volcanic 

 islands : nor is the grain or foliation (as we shall after- 

 wards see) any difficulty on the idea of the gneiss- 

 granite having been an intrusive rather than a meta- 

 morphic formation. The lines of mountains, but not 

 always each separate hill, range nearly in the same 

 direction with the foliation and so-called stratification, 

 but rather more easterly. 



On a bare gently inclined surface of the porphyritic 

 gneiss in Botofogo Bay, I observed the appearance here 

 represented. 



A frao-nient seven vards longr and two in width, 

 with angular and distinctly defined edges, composed of 

 a peculiar variety of gneiss with dark layers of mica 

 and garnets, is surrounded on all sides by the ordinary 

 gneiss-granite; both having been dislocated by a 

 granitic vein. The folia in the fragment and in the 

 surrounding rock strike in the same XXE. and SSW. 

 line ; but in the fragment they are vertical, whereas 

 in the gneiss-granite they dip at a small angle, as 

 shown by the arrows, to SSE. This fragment, con- 

 sidering its great size, its solitary position, and its 

 foliated structure parallel to that of the surrounding 



