The Volcanoes of Griqualancl East. 103 



The main plug of the vent consists of dolerite, which on sohdify- 

 ing has assumed a rough cohimnar structure, and further each 

 <3olumn has spht up into a number of wedge-shaped fragments. To 

 the south-east of this volcano there are a number of hills capped 

 with a precisely similar rock, distant from the volcano from 2 to 

 ■6 miles. These caps are prol^ably part of one of the early flows, and 

 if so it is one of the few that can be traced flowing southwards. It 

 ■can be easily seen how this particular flow would withstand denuda- 

 tion, owing to the hard, compact nature of the rock, the ordinary 

 lavas being easily decomposable amygdaloids. The petrographical 

 examination of this lava-flow promises many interesting features. 



The ordinary lava is an amygdaloid, and forms the bulk of the 

 ■crest of the Drakensberg range. It has been described by Professor 

 Cohen " from specimens sent from Basutoland by Mr. J. Orpen. 

 Professor Cohen calls the rock a melaphyre. I have, however, 

 •collected an immense number of varieties besides the melaphyre ; 

 the latter, though undoubtedly forming the larger proportion of the 

 lava-flows, is interbedded w4th other types of more acid or more 

 basic type, more crystalline or more glassy, of which I hope a prelimi- 

 nary description will be given in our next Annual Eeport. The lavas 

 :are full of blow-holes, as is usual in other lavas. These arise from the 

 fact that all molten material, as it exists deep down in the earth's 

 crust, is permeated with water-vapour, and owing to the great 

 pressure at these depths the vapour is forced to occupy a very con- 

 tracted space. When the lava is brought to the surface the same 

 thing happens as when a deep-sea fish is brought to the surface, the 

 water- vapour in the one case, and the air in the air-bladder of the 

 other, expands, and as the fish bursts with the expansion, so the 

 water- vapour forces the rock asunder and produces the cavities. No 

 rock in its natural state is, however, entirely free from water, and 

 this is always slowly creeping about, sucked in through cracks so 

 minute that the highest powers of the microscope fail to reveal 

 them, and even, as there is good reason to believe, through the very 

 inter-molecular spaces of the crystals. Water thus circulating- 

 carries with it certain salts in solution, and when it oozes slowly 

 into the cavities of the rock it deposits the salts before being driven 

 out again through the substance of the rock. In this way nearly all 

 the vesicles in ancient lavas are filled in with a group of minerals 

 peculiar to such rocks and called zeolites. Cohen has described the 

 zeolite occurring in his specimens as Heulandite, a hydrated calcium 

 iiluminium silicate, but I have not yet been al^le to determine the 

 varieties in my specimens ; besides the zeolites quaitz and calcite 



* Xcuex J(i}irhi(t']i, 187o. 



