gravels
of the hills and river valleys and accessible places in the river
bottoms of the Minas Geraes diamond fields have been worked for
diamonds, but there yet remains in the unworked portions of the streams
themselves, the greatest likelihood of the richest deposits of all.
Scientists
have thought that the matrix of the diamond in Brazil is itacolumite, a
kind of laminated granular quartz or ferruginous quartzose, and some
have claimed that the sandstone of the Grao Mogol district in which
diamonds have been found, was the matrix. The diamonds are found under
similar conditions, and in general, with certain companion minerals,
throughout the Brazilian fields, in what might be termed three tiers of
placement: in the heights: above the present water levels, and below
the water levels in the beds of the streams. In the heights, where they
are found in the itacolumite, the stones are not as plentiful, but they
average larger in size, and the edges of the crystals are not as
water-worn as those taken from the lower levels. On the hillsides of
the river valleys they are more numerous, more water-worn, and some of
the heavier companion minerals are not as plentiful. The sands and
gravels of the rivers yield even more diamonds, but they are usually
smaller, and are worn smoother. The minerals accompanying them are of
the lighter varieties. From these facts it is assumed that in the
ancient upheaval, the diamondiferous material was exuded through
fissures in the basic rocks, from which the rains of ages washed the
lighter pebbles to lower levels. From these deposits the smaller stones
were again rolled lower as the watercourses cut deeper into the
valleys, to the river beds of the present, each process being marked by
the increased
