Chap, iii.] MIXED VOLCANIC AND SAND ROCK. 6/ 



At about the middle of the northern coast of the main island 

 is a remarkable column-like mass of bare rock, which projects 

 to a height of 2,000 feet, and is known as the Peak. The 

 south-western extremity of the island runs out into a long 

 narrow promontory, which is composed of a narrow wall of rock. 



In tliis, at one spot near sea level, the sea has broken a 

 quadrangular opening through which it dashes in a cascade, 

 'rhis opening, known as the " Hole in the Wall," is visible 

 from a considerable distance at sea. At the opposite extremity 

 the island terminates in a low sandy point with sand dunes 

 upon it, beyond which stretch out the outlying islets already 

 referred to. 



The Peak forms a most remarkable feature in the aspect of 

 the island as viewed from the sea, and appears to overhang 

 somewhat on one side. One other hill in the island is 300 

 feet in height. The island is volcanic, but has evidently 

 undergone a vast amount of denudation, so as to obliterate all 

 traces of the centres of eruption. The Peak is composed of 

 phonolith, or clinkstone, as is also St. Michael's Mount, which 

 is a conical mass 300 feet in height. 



Rat Island and Booby Island are formed of a calcareous 

 sandstone, an .-I^^olian formation like that of Bermuda, but 

 here containing volcanic particles intermixed. This rock is 

 weathered in a closely similar manner to that at Bermuda, the 

 exposed surface being covered with irregular projecting pin- 

 nacles with excessively sharp honeycombed surfaces, in places 

 on Rat Island as much as two feet in height. 



On the western side of Rat Island, close to the shore, a 

 beach of large oval pebbles of phonolith is embedded in this 

 sand rock. In Platform Island the sand rock overlies columnar 

 volcanic rock. The main island is thickly wooded, and appears 

 beautifully green from the sea. 



The principal trees are what Webster, who visited the island 

 in 1828, calls the Laurelled Bara, which has dark green laurel- 

 like leaves, and an abundant milky juice, but the exact nature 

 of which is unknown, since I did not succeed in procuring a 

 specimen ; and a Euphorbiaceous tree, or rather tall shrub, 

 called by Webster, Jatropha or Pinhao {Jatropha gossypifolia). 



It has a pink flower, and at the time of our visit had only 

 single tufts of young leaves immediately beneath the inflores- 

 cence, although in full flower. Its bare stems and branches 

 render it a striking object amongst the green of the creepers 

 when the forest is viewed from the sea. Webster says that ic 

 casts its leaves in July and August, that is, at the commence- 

 ment of the dry season. It is evidently the tree mentioned by 

 Darwin as occurring on the Peak. 



