Page:The Journal of geology (IA journalofgeology11893univers).pdf/344
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 decomposition of granite rocks described by Count de la Hure and Capanema says:[1]
"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 contain beds of sandstone that are sometimes locally changed upon exposure to the hardest kinds 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.