6 A NATURALIST IN BRAZIL. 
summit is a very fine light-house, whose attendants, three 
men, are the only human inhabitants of the islands. The 
strata composing this island are inclined to the N. N. W., 
approximately, at an angle of 10°-15°, so that the island has 
Fig. 5. a slope to the northward, while on all 
, other sides it is precipitous. The sur- 

, trap, which is spread out over the other 
Section across island of ees 
Barbara, Abrolhos, a sales rocks, as is seen in Fig. 5. 
and ye 
=, andstones; b» Jt is a wonderful thing to see how 
rocks decompose and rot away in Brazil. Even gneiss and 
slate grow soft to a depth sometimes of even a hundred feet. 
This trap-bed at the Abrolhos is decomposing also, but this 
takes place in a very interesting way. The trap, which is 
a very hard and heavy dark-bluish rock, is cracked up on 
the surface into angular pieces of all 
sizes, as represented in Fig. 6. 
If the rock were smooth and un- 
broken on the surface, it would de- 
compose only on the upper surface, 
ik Witter sea in  fivouwli these cracks, and each fragment 
decomposes all around, so that a concentric coating of rotten 
rock is formed (Fig. 6, b), which may afterward be removed 
by rains. Thus each piece loses coating after coating like 
the layers of an onion, becoming ever more rounded in form 
as this goes on, until at last the surface of the bed is covered 
over with rounded boulder-like masses, often resembling can- 
non balls (Fig. 6, a). Nearly the whole surface of the island 
of Santa Barbara is covered by these rounded masses of trap. 
The vegetation of the island is very scanty, and, save a 
Siriba on the island of that name, to which bear company 
two dwarf cocoa palms, trees there are none. Several species 
of coarse grass abound, and give sustenance on Santa Bar- 
bara to a herd of many handteds of goats. There are some 
thickets of dwarf mimosas, and a few ferns, etc. 

a face is mainly composed of a bed of 


