258 J. C. BRANNER — DECOMPOSITION OF ROCKS IN BRAZIL. 



sents. I suspect that his measurement was a vertical one and not made 

 at right angles to the rock face. 



Professor Hartt makes many references to the deep decomposition of 

 the rocks, though he seldom gives any figures on the subject. Inasmuch 

 as he was of the opinion, when he wrote his book upon Brazilian geology,, 

 that that country had been glaciated, the materials called drift by him 

 may usually be put down as decomposed rock in place or but little dis- 

 turbed.* 



George Gardner, the English botanist, mentions 30 to 40 feet of clay 

 about Rio de Janeiro.f 



Mr Darwin J says that — 



"near Rio every mineral except the quartz has been completely softened, in 

 some places to a depth little less than 100 feet. ... At Rio it appeared to me 

 that the gneiss had been softened before the excavation (no doubt by the sea) of 

 the existing broad, flat-bottomed valleys. ... At Bahia the ^neiss rocks are 

 similarly decomposed." 



On the Xictheroy side of the bay there are several exposures of decayed 

 and half-decayed rocks on the high headland near the northern end of 

 the Jurujuba bay. Here the waves have undermined the materials, but 

 while they are still hard enough to stand in vertical cliffs, they are far 

 from being as hard as the ordinary undecomposed gneiss. These cliffs 

 are in places as much as 100 feet in height. 



Along the railway leading from Nictheroy to Nova Friljurgo gneiss is 

 the rock of the country, and it is deeply decomposed along the whole 

 route, showing here and there the dome-like peaks so characteristic of 

 this coastal region. At Cantagallo this decomposition is quite as marked 

 as it is at Rio de Janeiro. One of the exposures shows a hillside where 

 disintegration has penetrated more than 100 feet, and as the rocks at the 

 bottom are quite as soft as those at the top it seems safe to presume that 

 decomposition has extended still deeper. 



The rocks through the region from Cantagallo to Campos are all 

 gneisses and granites, and show everywhere the same deep decomposi- 

 tion as that about Rio de Janeiro. 



Petropolis, in the Organ mountains, is in a region of gneiss, and as the 

 valleys about it are narrow the sight of the deep cuts in the decomposed 

 rocks is a common one there. Others are exposed along the railway lead- 

 ing up from the foot of the Serra. 



Good cuts in decomposed gneiss are exposed along the line of the Cen- 



* Geology and Physical Geography of Brazil. Ch. Fred. Hartt. Boston, 1870, pp. 25, 28, 31, 

 340,508,509,564. On Hartt's change of views on the subject of glaciation see "The Supposed 

 Glaeiation of Brazil." J. C. Branner. Journal of Geology, vol. i, 1893, pp. 753-772. 



t Travels in the Interior of Brazil. George Gardner. London, 1846, p. 11. 



X Geological Observations. Charles Darvvrin. 2d ed., London, 1876, pp. 427, 428. 



