98 O. A. Derby— Geology of the Diamond. 
the localities sa the two quartzites can be seen in juxtaposi- 
tion and where they are at the same time clearly distinguishable 
one from the hee are few, this unconformability has ie 
probably belongs to the newer series, and that the diamond en- 
tered into its com position ready-formed like any other pebble. 
The locality of Sao Joaio da Chapada, where the diamond has 
been mined in clay (barro), is described. It is shown that the 
mine is excavated in the undisturbed soft material resulting 
from the decomposition of beds of unctuous schists underlying 
a bed of quartzite (itacolumite) which appears at the entrance 
of the mine. The diamond-bearing material was not exposed 
in situ, but two masses dislocated by slides were pointed out by 
a negro, who knew the mine thoroughly, as the diamantiferous 
barro. One consisted of a black clayey mass which, on a fresh 
fracture, revealed thin alternating layers of white clay resem- 
bling lithomarge and black pulverescent i iron oxide, The other 
mass consisted of a portion of a quartz vein, the quartz being 
much fractured ey traversed by brilliant, ‘plates of ee aa 
an abundant black sand, consisting for the most part of micro- 
scopic tourmalines, according to the Herp Ni of Professor 
J. W. Mallet. Small hexagonal crystals a also described by 
H. Rose in a mass nS barro containing a ie obtained at 
Sao Joao by Messrs. Heusser and Claraz. It was concluded 
from these observ pens that the diamond occurs at Sao Joao in 
its original matrix, and that this matrix is a vein of quartz 
bce psec by a rock of unknown nature, but containing iron 
tourmalines traversing the series of unctuous schists and 
jeeeseniaes 
Since the ‘publication of these papers, both Professor Gorceix 
and myself have revisited the diamond region, and these views 
have been fully confirmed. A specimen of rock from Grao 
Mogol, obtained through the kindness of Dr. Cartaio Jardim, 
shows distin netly by the side of a diamond a rounded water- 
worn pebble, and Professor Gorceix was so fortunate as to eX- 
tract, under his own immediate supervision and with all neces- 
sary precautions, several diamonds from the barro of the 5ao 
Joao mine. 
Near Diamantina I examined a mine in a rotten conglomer- 
