232 THE VOLCANIC NECKS 



now builds up the higher portion of the ridge and a 

 great bulk of rock that has disappeared under the cease- 

 less attack of the weather. 



The largest of the Matatiele pipes is on the farm 

 York ; it is about a mile in diameter, and it has been cut 

 in two by a tributary of the Mabele Eiver. The vent is 

 filled partly with amygdaloidal and doleritic lavas, and 

 partly with agglomerate. The dolerite was the first 

 rock to flow from the pipe, and it is still connected with 

 a columnar flow of dolerite that lies upon the Cave 

 sandstone. The doleritic rock was succeeded by amyg- 

 daloidal lavas, part of which are still preserved in the 

 lava-flows, 4,000 feet thick, near Ongeluk's Nek. Near 

 the volcano the lava contains large masses of sandstone 

 and shale, baked and converted into porcellanite by the 

 heat of the lava. There are some baked shales that Prof. 

 Schwarz regards as having been formed in temporary 

 lakes or streams on the volcano itself, and subsequently 

 hardened by fresh flows of lava. Brown, gritty soil is 

 preserved between some of the lava streams that issued 

 from this vent, indicating that the volcano, even if it 

 started its activity below r the water level, piled up its 

 lava sufficiently to form a land surface. The agglomer- 

 ate is dark blue in colour, and includes large numbers 

 of fragments of lavas and sedimentary rocks ; this 

 material is probably the result of the final explosive out- 

 burst of the volcano. Evidence of the long duration 

 of the activity of this vent is given by the old valleys 

 carved out of some of the lava-flows and filled in by later 

 ones. 



Taking into consideration the great thickness of lavas 



