864 Fauna of the Islands of Fernando de Noronha. 
the walls of thatch, in the roof, among the pots and pans—every- 
where. No sooner did I lie down in my hammock than they made 
their way down the cords and into my bed. During the early part. 
of the night I amused myself by allowing them to reach the middle 
of the taut hammock cords, when, by striking the cords a sharp 
blow, the vibrations would shoot them off into space. This ceased 
at length to be amusement, and when, late in the night, I occasion- 
ally fell asleep, it was only to be awakened in a few moments by 
the mice nibbling at my face or hands or feet, or by their falling 
upon me from the roof. 
The occurrence of rats upon the main island and of mice only 
upon Ilha Rapta may possibly be explained by the mice having 
been imported to the smaller from the larger island. The geology 
of this group of islands, however, suggests another plausible expla- 
nation. Fernando de Noronha and the small islands lying about 
the main one are of igneous rocks, with the exception of some 
limited exposures of comparatively recent calcareous sandstones 
formed by the consolidation of sand dunes. These calcareous sand- 
stones form all of Ilha Raza and Ilha do Meio, the southwest third 
of Ilha Rapta, and overlie a portion of Sao José and the extreme 
northeast point of the main island. -It occurs also at the southeast 
base of Atalaia Grande, and in the Bahia de Sudoest, where it 
forms Ilha de Chapeo and the shore of the bay in places. 
In all these localities the sandstone is cut away on its south- 
eastern side, and an abrupt or overhanging face is exposed to the 
ocean’s surf, while its upper surface stands at an elevation of from 
thirty to fifty feet and more above the water. On the landward 
side of the exposures on Ilha Rapta, Sao José and the main island, 
these sandstone beds thin away to the west. These facts, and others 
_which need not be mentioned here, go to show that the south- 
eastern coast of these islands formerly extended much farther 1n 
that direction, and that the calcareous sands, of which these rocks 
are formed, were blown inland from the beach which once existed 
in that direction. The ocean, however, has gradually encroached 
upon the island, and especially from the east, until what was 
formerly one island has been separated into six, namely, Sao José, 
Sella Ginete, Ilhas do Meio, Rapta, Raza, and the main island. 
The mice which are so abundant upon both islands now may 
