









V\ 





AND CHATHAM ISLAND. 



157 



102.— Scoriaceous rock, occurring near veins of sili- 



98) 



filled with fine concentric layers of white chalcedony which 



'/ 



103.— Siliceous conglomerate, with small prismatic 



crystals of tourmaline and crystals of quartz, and coated 

 with a thin layer of siliceous sinter. 



EOCK RESEMBLING SYENITIC GNEISS, probably from 



one of the laminated beds alternating with and passing 

 into obsidian, noticed in Darwin « On Volcanic Islands " 



104. 



pp. 56 and 57. 



105, 



from pumiceous tuft 



very tough, compact, pale brown stone, with a smooth and 

 even fracture, and containing a small proportion of car- 

 bonate of lime. Some of the larger concretions are des- 

 cribed as mere shells filled with slightly consolidated ashes, 

 (bee Darwin " On Volcanic Islands," p. 47 



) 



Volca 



nic Specimens from the Galapagos Archipelago, 



Chatham Island. 



The Galapagos Archipelago consists of ten islands, si- 

 tuated under the equator, 500 or 600 miles westward of 

 he coast of America. They are all formed of volcanic 

 rocks, and are chiefly remarkable for the immense number 



craters with which they are covered. These are formed 

 either of lava and scoriae, or of tufa ; in the latter case they 

 Pesent beautifully symmetrical forms, which appears to be 

 ^ving to their having been formed while standing out at 

 sea by eruptions of volcanic mud, without any lava 



tha tham Island is the largest of three islands, intersected 



1 &;* ° f 43 ° 46 " S " and bj the meridian of 



chieflv J C0 ^ inS 47? SqUare miIes ' The rocks ^e 



the Zlr n \ C '. and lt PreSGntS a r "^ ed ' arid Wearance ; 



bel! „ ?l tlC laVa ' ° f Which the surfoce " composed, 

 °eing covered by a dwarfed and parched brush-wood. 



H. W. Bristow. 



Upper 

 Galleky. 



Wall-case 2, 



