14 Transactions of the South African Philosophical Society. 



southern angle, and a conical mound of 700 feet rises in the south- 

 west, the two heights being separated by a V-shaped ravine. 



Nightingale Island is the smallest of the Tristan d'Acunha group, 

 lying towards the south. It is surrounded by rocks, amongst which 

 there are two islets measuring one-half by one-sixth of a mile. One 

 of these, Middle Island, is 150 feet high, with an undulating summit. 

 The second islet, which also lies to the north of Nightingale, is 

 Stoltenkoff Island, and has a height of 325 feet. Nightingale Island 

 is a mile long from east to west, and about three-quarters of a mile 

 broad. A channel, ten miles wide, and over 465 fathoms deep, 

 separates Nightingale from Inaccessible, while depths beyond 1,000 

 fathoms occur in some places between Nightingale and Tristan 

 d'Acunha. 



Nightingale differs greatly in appearance from the other islands of 

 the group, being more varied in outline and surrounded by cliffs 

 only thirty or forty feet high, and often less. The southern part of 

 the island is more picturesque, the ground rising by successive crests 

 to a peak 1,105 feet high, one side of which is almost vertical for half 

 its height. The rest of Nightingale is undulating, and the rocks, 

 except at a few isolated points, are covered with verdure. 



No traces of recent volcanic activity are to be seen. The rocks 

 consist chiefly of a conglomerate or breccia of doleritic fragments 

 embedded in a whitish felspathic mass. Here and there the con- 

 glomerate is surrounded by beds of volcanic rock probably of more 

 ancient origin. Marine erosion has hollowed the cliffs girdling the 

 island into innumerable caves, which are situated a little above sea- 

 level, and prove that the island has been recently elevated. A raised 

 beach on the top of the cliffs confirms this supposition.* 



The volcanic conglomerate is a phonolitic tuff. The bluish grey 

 rock is speckled with white kaolinised patches ; the ground mass is 

 waxy and considerably altered, and is impregnated with limonite 

 in some places. Under the microscope, the mass is composed of 

 minute sections of nepheline, and in their arrangement show a 

 well-marked fluidal structure. Microliths of augite are associated 

 with the nepheline ; sanidine, plagioclase, and hornblende are also 

 present. 



In this phonolitic mass there are embedded heterogeneous clastic 

 fragments, which prove the tufaceous origin of the rocks, forming 

 almost the entire explored portion of the island. 



Eruptive rocks of an andesitic type like those of Nightingale are 

 also present, together with vesicular hornblende andesites. 



What the heterogeneous rocks contained in the tuff are is not 



* Buchanan, Proc. Roy. Soc, vol. xxiv. p. 614. 



