O, A. Derby—Decay of Rocks in Brazil. 139 
Then come 60 meters of clays (decomposed shales) very simi- 
lar to those above described, and evidently belonging to the 
same series, although it is impossible to exactly codrdinate the 
beds of the two borings. This, however, is not surprising, as 
such thin and rapidly alternating beds are not likely to preserve 
the same order and relative thickness over any considerable 
area. At the depth of 80 meters a gneissoid rock is met with 
which is quite as much decayed as the overlying shales. This 
rock appears to have been a highly micaceous gneiss contain- 
ing a few scattered crystals of feldspar which are completely 
kaolinized. The mica is in part completely altered to a bluish 
unctuous clay, part is only partially decomposed, presenting ill- 
defined flakes with a silvery luster, while occasional scales o 
black and unaltered mica can still be distinguished in the rock. 
The mass effervesces slightly with acids and has apparently 
received a portion of lime from the overlying shales which con- 
tain caleareous layers. j 
ar as can be judged from the small fragments taken from 
the boring, this mass is a true gneiss decomposed in situ and 
not a bed of arkose intercalated in the shale. The shale bed 
immediately above it, about a meter thick, is homogeneous 
and of a dark umber color, without any appearance of having 
