1/ ry h 



THE 



JOURNAL OF GEOLOGY 



JULY-AUGUST, i8g6. 



DECOMPOSITION OF ROCKS IN BRAZIL. 



Under the above title Dr. J. C. Branner'' has recently brought 

 together an extensive series of observations scattered through 

 the literature relating to Brazil, with many original notes of his 

 own bearing on the subject of rock decay in that countr}^ and 

 the causes of the exceptional character and depth attributed to it 

 by most writers. While appreciating fully the value and interest 

 of the facts collected in the said paper, the present writer is 

 somewhat inclined to question the hypothesis that furnished the 

 motive for the collection, that is to say, the hypothesis that rock 

 decay in Brazil is of an exceptional character requiring for its 

 explanation the operation of special causes. The subject is one 

 in regard to which he has felt great diffidence in formulating an 

 opinion, even for his own private use, from his lack of familiarity 

 with other regions of like petrographical and geological struc- 

 ture but situated under different climatic conditions, with which 

 alone the comparison can properly be made. As it is probable, 

 however, that in this respect he is in the same boat with many of 

 the writers whose observations are relied upon for establishing 

 the major premise in the case, he ventures to present some of 

 his own observations on certain aspects of the subject. 



Professor Pumpelly treating of the secular decay of rocks ^ 

 has well remarked that "The depths of this deca}^ other things 



' Bull. Geol. Soc. of Am., Vol. VII, p. 256. 

 'Bull. Geol. Soc. of Am., Vol. II, p. 210. 

 Vol.. IV., No. 5. 529 



