Notes on a Visit to Ftrnando Noronha. 429 



a reef of rock, composed of sand, coral, tubiculous lamelli- 

 branchiata, and calcareous algae, is seen, at low water, almost 

 completely surrounding the island. It is exceedingly rugged 

 and full of holes, very like some presented in the sand rock 

 into which it passes, without any line of demarcation what- 

 ever, in a number of different places. Egg Island is united 

 to the mainland by the reef, so as to be accessible at low 

 water, as is Platform Island, but, in the latter case, the reef 

 is replaced by basaltic boulders. It would thus appear that 

 the sand rock is rather raised reef than consolidated dunes. 

 A very friable volcanic conglomerate occurs on the seaward 

 slopes of the East Hills, and also on the declivities above the 

 south coast near the west end, and nearly opposite on the 

 north-west side, and on the slope toward the Lapate, Some- 

 thing very similar appears in the bed of a stream in the 

 central region. The faces of the shore cliffs in the western 

 part of the mainland, and also in Rat Island, show bands of 

 rock something between clay and conglomerate, the ground 

 mass being highly vesicular, but containing a number of 

 round pieces of solid basalt. This sometimes gives the cliffs 

 a terraced form, and sometimes, when seen from a distance, 

 makes them appear stratified. Besides the phonolite in the 

 centre of the island various bosses of that rock crop up 

 through the basalt. It appears thus at the base of the East 

 Hills, and also on the north-west coast on both sides of 

 Water Bay. Moro da Villa is phonolite, as is the point of 

 land immediately adjacent to it, and the high bluff on which 

 the fort stands, on the east side of the bay. Another phono- 

 litic boss occurs between the fort and the north-east point, 

 and, on the south coast, a small hill, known as Moro Branco 

 from its white colour, is composed of the same rock. Of the 

 outlying islands St Michael's Mount, Les Clochers, and the 

 other rocks along the south coast are phonolite, but the 

 Twins, to the north, and Platform and Eat Islands are of 

 basalt. Wherever the phonolite and basalt meet, both rocks 

 show traces of alteration, the former becoming quite dull and 

 of a pale grey, almost white, while dykes of the latter, of 

 which there are often a number traversing the phonolite at 

 such places, contain abundant crystals of augite, but, so far 



