THE KARROO SYSTEM 229 



variation. The smallest one known was found in 

 Matatiele, and is only four yards across; the great 

 majority exceed 100 yards in diameter, while there 

 are a few which are as much as a mile wide. Some 

 are circular in outline, while others again are much 

 elongated. 



In their contents also the pipes vary greatly. A few 

 necks, mostly small ones, are plugged entirely with 

 doleritic lava ; a larger example is to be found close to 

 the Gatberg in Elliot. 



Many of the pipes are now filled with a light or dark 

 bluish agglomerate containing fragments of lavas and of 

 sedimentary rocks, the latter being usually derived from 

 the Stormberg beds, though Pre-Karroo quartzites are 

 occasionally represented. 



The matrix of these agglomerates is largely composed 

 of grains of quartz and felspar, derived from a sediment- 

 ary or a granitic rock, as both orthoclase and microcline 

 are abundant, along with zircon, rutile, hornblende, 

 garnet, muscovite, and tourmaline ; these are minerals 

 that do not occur in the lavas. With them are others, 

 plagioclase felspar especially, which are important con- 

 stituents of the basalts, and of which both small and 

 large fragments are frequently abundant in the agglom- 

 erates. Pieces of charred wood have also been found ; 

 they may be the remains of trees which grew on the 

 slopes of the volcanoes during periods of quiescence. 

 On a renewal of activity fragments of these trees fell 

 into the crater and became embedded in the breccias. 



Not uncommonly finely divided sedimentary material 

 so predominates that the tuff filling the neck closely 



