VOLCANIC PIPES B51 



silica. This has happened chiefly in the smaller pipes 

 and in the peripheral portion of the large vent ; a similar 

 process has caused the hardening of the shales and 

 sandstones at their contact with the vents and dykes. 

 The carbonates, sulphates, oxides of iron and silica were 

 probably carried to their present position by water as- 

 cending the channels of eruption after the period of vio- 

 lent activity had closed ; their deposition may be regarded 

 as analogous to the effects of the " solfataric " stage of 

 recent volcanic areas. 



The smaller necks in the Saltpetre Kop area do not 

 materially affect the regularity of the qua qua-versal dip 

 about the central vent. 



In no other vent of the kind we are dealing with in 

 this chapter is the outward dip or up-turning of the 

 surrounding strata so strongly marked as in the case of 

 the central neck of the Saltpetre Kop group. Wherever 

 the strata in immediate contact with one of the pipes 

 are exposed, and have been examined with attention, 

 they have been found to dip away from the contact, as 

 though the ascent of the materials filling the pipes had 

 bent the edges of the strata upwards. This has been 

 noted at some of the Sutherland Commonage vents, at 

 Balmoral (Eatel Fontein), at Matjes Fontein, Schiet 

 Fontein and at Kirnberley. This feature seems to be 

 peculiar to these vents, for where notice has been taken 

 of the dip of the strata near the pipes of volcanoes of 

 the more usual types, the strata have been found to be 

 inclined towards the pipe as though dragged downwards 

 by the settling of the contents after the activity of 

 the volcanoes ceased. 



