766 THE JOURNAL OF GEOLOGY. 



than a score of statements of a similar nature may be cited from 

 Liais' book. 



Count de la Hure has also pointed out how diorite breaks up 

 into boulders, and cites in evidence some of the very cuts on the 

 Pedro II. Railway which Agassiz and Hartt refer to the drift. 

 Saldanha da Gama in speaking of the exfoliation and decom- 

 position of granite rocks described by Count de la Hure and 

 Capanema says : J 



" This and many other facts gathered by the Brazilian naturalist in his 

 observations on diorite and other rocks of that class led the eminent Swiss 

 geologist to point out that the study of the drift in Brazil will not be well 

 understood so long as one hasn't a thorough knowledge of the decomposition 

 of the rocks." 



He also refers to the fact that these phenomena may be 

 observed in several of the Brazilian provinces. 



The two kinds of boulders above mentioned are common in 

 the regions of crystalline rocks ; a third kind is found in those 

 parts of eastern Brazil that are covered, or were formerly covered, 

 by Tertiary sediments, namely in the State of Bahia, and thence 

 northward to the Amazon valley. These Tertiary deposits con- 

 tain beds of sandstone that are sometimes locally changed upon 

 exposure to the hardest kind of quartzite. Most of the 

 associated beds are friable and easily eroded, so that when the 

 surrounding strata have been removed there are left behind a few 

 blocks of quartzite, varying in size from a foot to four feet in 

 diameter. These boulders are so unlike the rocks from which 

 they have been derived and by which they are surrounded, that 

 unless one has given special attention to the study of Tertiary 

 sediments in that region he is liable to be much puzzled and even 

 misled by them. 2 



ORIGIN OF THE WATER - WORN MATERIALS. 



The second class of evidences by which Agassiz and Hartt 

 were misled consisted of transported, water-worn materials. 



J Revista do Instituto Historico do Brazil, 1866, XXIX., 421 et seq. 

 2 See Branner's Cretaceous and Tertiary Geology of the Sergipe-Alagoas Basin 

 of Brazil. Trans. Amer. Phil. Soc, XVI., 1889, 419-421. 



