chap. xin. Rio de Janeiro. 425 



angle of fifty degrees. Hence all the observations 

 hitherto made tend to show that the gneissic rocks 

 over the whole of this part of the continent have their 

 folia extending generally within almost a point of the 

 compass of the same direction. 1 



Rio de Janeiro. — This whole district is almost 

 exclusively formed of gneiss, abounding with garnets, 

 and porphyritic with large crystals, even three and four 

 inches in length, of orthoclase feldspar : in these 

 .crystals mica and garnets are often enclosed. At the 

 western base of the Corcovado, there is some ferruginous 

 carious quartz-rock ; and in the Tijeuka range, much 

 fine-grained granite. I observed boulders of greenstone 

 in several places ; and on the islet of Villegagnon, and 

 likewise on the coast some miles northward, two large 

 trappean dikes. The porphyritic gneiss, or gneiss- 

 granite as it has been called by Humboldt, is only so 

 far foliated that the constituent minerals are arranged 

 with a certain degree of regularity, and may be said to 

 have a ' grain,' but they are not separated into distinct 

 folia or laminas. There are, however, several other 



1 I landed at only one place north of Bahia, namely, at Pernam- 

 buco. I found there only soft, horizontally stratified matter, 

 formed from disintegrated granitic rocks, and some yellowish im- 

 pure limestone, probably of a tertiary epoch. I have described a 

 most singular natural bar of hard sandstone, which protects the 

 harbour, in the Appendix to my work, ' The Structure and Distribution 

 of Coral Beefs,' 2nd edit. p. 265. 



Abeolhos Islets, lat. 18° S. off the coast of Brazil. — Although 

 not strictly in place, I do not know where I can more conveniently 

 describe this little group of small islands. The lowest bed is a 

 sandstone with ferruginous veins ; it weathers into an extraordinary 

 honey-combed mass ; above it there is a dark-coloured argillaceous 

 shale ; above this a coarser sandstone — making a total thickness of 

 about sixty feet ; and lastly, above these sedimentary beds, there is 

 a fine conformable mass of greenstone, in some parts having a 

 columnar structure. All the strata, as well as the surface of the 

 land, dip at an angle of about 12° to N. by W. Some of the islets 

 are composed entirely of the sedimentary, others of the trappean 

 rocks, generally, however, with the sandstones cropping out on the 

 southern shores 



