40 



THE VOYAGE OF H.M.S. CHALLENGER. 



is worth while noting that, although he wrote at a time when our knowledge of crystalline 

 rocks was in its infancy, the main features of his system remain unaltered. It is right 

 to add that Darwin had been preceded at Ascension by Lesson, who had already given 

 pretty precise indications of the nature of the rocks of this island. 



The Island of Ascension is situated in the South Atlantic Ocean, in latitude 8° S., 

 and longitude 14° W. ; according to observations made by the officers of the Challenger, 

 the central summit is in latitude 7° 56' 58" S., and longitude 14° 20' W. The form of 

 the island is an irregular triangle, each side measuring about 6 miles ; it is 7\ miles 

 long and 6 miles wide. The surface is very irregular, and on a general view appears 

 sterile and miserable in the extreme, presenting an expanse of black, burnt rocks, 

 unrelieved by the least vestige of soil. The highest point of Green Mountain, situated 



The Green Mountain and Extinct Craters, Ascension Island. 



in the east of the island, rises to 2870 feet above the sea, and from the summit one sees 

 forty or more little peaks scattered about in all directions. The accompanying woodcut 

 will give some idea of the appearance of the island, which is entirely volcanic, 1 and in 



1 Lesson, in bis description of Ascension, states his belief that the island is formed of a single volcano, the dejecta 

 from which built up Green Mountain. "All the other eminences which rise to the north and on the plateau of the 

 island without regular order, either as isolated cones or in groups, are more recent volcanic openings, the craters of 

 which, symmetrically formed as a rule, are directed towards the principal volcano, Green Hill, on the side of the 

 prevailing wind, producing a steep declivity in this direction. These fire - breathing mouths are very regularly 

 characterised in the secondary mountains of Ascension, but less so in those of Cross Hill, Red Hill, Zebra Hill, etc. ; 

 the greater number present craters in a state of perfect completeness. Green Hill derives its name from the verdure 

 of a vigorous growth of plants upon its summit. The vegetation ceases at the lower third of the mountain, which is 

 composed of naked rock piled up confusedly according to the fractures it has undergone. All the other mountains 

 are quite bare, covered with ferruginous scoriae of a prevailing red colour. The surface of the island is composed of a 

 detritus of trap and trachyte pulverised and deposited here and there in beds of small extent, bordered everywhere 



