BRANNER: THE STONE REEFS OF BRAZIL. 51 
which are larger than one’s fist. Mingled with the quartz are also, both 
here and elsewhere over the reef’s surface, large pebbles of dark red 
Cretaceous or Tertiary sandstone such as occurs in the hills that skirt 
this coast. There are also occasional patches and lumps of a very black, 
compact, heavy rock made up of grains of titaniferous iron. At one place 
near Mamanguape Point a piece of this black iron rock, 4º x 4! x 5!, was 
found on top of the reef cemented compactly to an underlying bed ot 
Coarse white sand and pebbles. This rock is as black as coal, is com- 
posed of particles of black titaniferous iron sand, and shows false bed- 
ding. A few other blocks of similar material, one foot square, are near 
this one and attached in a similar fashion. Half a mile further south 
angular and subangular fragments of this rock the size of the two fists 
and some as big as one’s head are buried in the sugar-brown rock of the 
reef, forming a sort of breccia for fifteen metres or more. At many other 
places this material occurs ав pebbles scattered through the brown reef 
rock. These spots are caused by the concentration of titaniferous iron 
sands upon the ancient beaches. They are cemented by carbonate of 
lime and magnesia in the same manner as the other reef rock. 
The most characteristic thing about the rock of this and of all the 
other stone reefs is the presence in them of fossil shells of various mol- 
lusks now living along the coast. These shells are not evenly distributed 
through the rock, but are abundant in some layers and almost or quite 
wanting in others. Most abundant of all is a small, beautifully varie- 
gated, thick-shelled Venus known here as mariscos. These shells still 
retain in the rock their brilliant colors. The shells are never found in 
pairs аз in life, but broken apart and with the horny cuticle they have 
when alive worn off. During their lives these mariscos burrow in the 
sand of sandbars and protected sandy beaches to a depth of about two 
inches, They are edible, and are used for food more or less all along 
the coast, 
In one of the blocks near the northern fourth of this reef was found 
also a block of Porites, one of the hardy corals now growing upon the 
Coral reefs and in the rocky tide-pools of the coast. 
The reef rock proper when found in large slabs or projecting points 
rings under the hammer almost like bell-metal. It is, however, not 
everywhere equally hard : the upper beds, especially those exposed now 
and then to the sun and atmosphere, are as a rule hard and even 
(uartzitic in fracture, while in other places the same beds may be 
rather soft and incoherent. 
The surface features of the reef are not without interest. Here and 
