CHAP. II.] MADEIRA TO THE COAST OF BRAZIL. lOY 



might we not catch it, but he said he would prefer that we 

 should not. The governor of a convict establishment is in a 

 very delicate position, and bears a heavy responsibility, not un- 

 accompanied with serious risk, and it is, of course, difficult to 

 judge his conduct in such a case ; but it is not easy to see why 

 his determination should have been exerted against our throw- 

 ing light upon the natural history of the island only. Captain 

 Nares and a party visited St. Michael's Mount and " Eat '' and 

 "Platform" Islands. Mr. Moseley collected a great many 

 plants, and Mr. Buchanan made some observations on the geo- 

 logical structure of the islands, which I quote from his notes : 



" The highest island, St. Michael's Mount, forms one of the 

 prominent peaks which are characteristic of the group. It is 

 very steep and formed entirely of phonolite, which occurs co- 

 lumnar at the base and massive toward the top ; on the western 

 side, where we landed, the columns are inclined to the horizon 

 at an angle of about 30°. Their transverse section looks nearly 

 square, the corners being, however, considerably rounded off. 

 The columns are for the most part slender, and their mass is of 

 a dirty green color. In this the glassy feldspar crystals are 

 arranged with great regularity, with their broadest faces in a 

 plane perpendicular to the length of the column. The steep 

 sides of the Mount are covered with loose blocks of massive 

 phonolite, fallen down from above and retained in position on 

 a very steep incline by the stems of most luxuriant creeping 

 plants. On the weathered sides of these blocks the glassy 

 feldspar crystals, and also the crystals of hornblende, though in 

 a less degree, project sometimes to the extent of a quarter of 

 an inch, so much more decomposable is the crypto-crystalline 

 matrix than the crystals occurring porphyritically in it. This 

 rock possesses in an eminent degree the characteristic property 

 from which it derives its name : when struck with a hammer, 

 it literally rings like a bell. 



" The rock is cleft from top to bottom in two planes nearl}' 



