422 A YEAR IN BRAZIL. 



our own limestone caves, and as I was unable to visit the metalli- 

 ferous districts of Pitanguy, Ouro Preto, or Sao Joao del Rey, 

 though these were all within a ride of a few hours, I shall con- 

 fine my remarks to the gneissic formations and their decomposi- 

 tion. 



Stratified gneiss forms the total mass of the soil in all elevated 

 parts near Rio de Janeiro and over vast areas of the empire of 

 Brazil, the vegetable earth resting immediately on the top of these 

 strata. I observed the same strata at Bahia. Gardner found it in 

 the province of Ceara, five hundred miles to the north of Bahia ; 

 and " Humbolt describes the gneiss-granite over an immense area 

 in Venezuela, and even Columbia." * The mineral composition of 

 gneiss which is made up of quartz, felspar, and mica is the 

 same as the components of granite, the only difference is in the 

 former's foliated texture, hence gneiss may be described as schistose- 

 granite. The gneiss is always upheaved at a high angle, and though 

 the base is felspathic, the different strata have important structural 

 differences in composition ; granite and hornblende are present in 

 different localities, and modify very considerably the composition 

 of the rocks. To enter into the lithological structure of this group 

 of strata is beyond the scope of a brief article ; I should have to 

 describe porphyries, granite, syenite, diorite, pegmatite, eurite, 

 quartzite, gneiss, mica schists, magnetic ironstone, garnets, murchi- 

 sonite, kaolin (porcelain clay), and numerous other combinations 

 of quartz, felspar, mica (red, black, or yellow), hornblende, etc., 

 with other metamorphosed sedimentary deposits. 



The most salient characteristic of these gneisses is their con- 

 dition of decomposition, which has been effected on an immense 

 scale. Even the United States do not present such an intensity 

 of remarkable phenomena as do the rocks of Brazil. It is not 

 unusual to find the gneiss completely transformed into clays to a 

 depth of over three hundred yards. The barrancadas or canyons, 

 carved out by the rains, give the plainest evidence of this de- 

 composition. The heavy rains, especially after long continuance 

 of dry seasons, produce the most marvellous effect. As an 

 instance, I may cite (Liais) the storms of March, 1859, when five 

 inches and a half (fourteen centimetres) of rain fell at Rio de 

 * " Geolog. Observ.," Chas. Darwin, p. 424. 



