O. A. Derby— Brazilian specimens of Martite. 3738 
é. non length of time during which the waters stood a 
m height was probably less than five years. This 
a i with the following facts: that the terraces marking this 
ates evel have much less extent than those of lower levels: 
that they are confined more narrowly to the vicinity of the 
heights by 10 or ee ~ 8 a wide range of evidence of this 
may yet be made 
e sacar Nae length of the era occupied by the rise 
and fall of the flood is not a question within the range of this 
paper, and does not appear to admit of determination by any 
of the facts considered. 
Art. XXXIX.—On nd azilian specimens of Martite; by 
ORVILLE A. DERBY. 
THE opinion has lately been Bes (M. Gorceix, Comptes 
Rendus de l’Acad. des Sci., No. 7, 1880; Annals da Escola de 
Minas de ‘aes ee No. 1), ‘bbe the octahedral crystals of 
oligiste, socommon in the metamorphic schists of Minas Geraes 
and known by naa name 9 martite, are due to the transforma- 
tion of pyrites. That this is true of some of the erystais, is 
beyond a t examination of fine collections 
4 doubt, but 
from typical localities indicates that a large portion shoul 
rather be considered as produced by the alteration of magnetite. 
collected by myself from a single locality neat the village 
of Itambé, rows a partially decomposed quartzose micaceous 
schist, oe were attracted and for the most ‘ae t freely lifted 
shoe msehch and 251 showed the same indifference to the bar 
magnet. 
many places to the north, i the wip sprain of the es terraces 
(indexes of the so-called “‘Kames.”) The es of such ice-floes would have 
been largely from northern pay ehatter.4 disnchidin of the s tream con- 
The liability of floating masses strand, and to drift up stream as well as 
down, in ESC of eddies slong the sides of a river, is illustrated in an in- 
structi n the Connecticut, in early summer , by t great crowd (sometimes 
rnillions together) of ve (reetrunks) that float down it to lumber-yards in Mas- 
chusetts and farther ere it not for a slong bs of hands to look out 
for the esta an ater’ eep isein moving, they would be generally — before going 
far on their course. The bends in the stream add much to the difficulties. 
