THE KARROO SYSTEM 225 



6. Lavas 130 feet 



5. Red sandstones and shales . . . . 20 ,, 



4. Lavas 70 ,, 



3. Shales 40 



2. Lavas 630 



1. Cave sandstone ...... 100 



The red sandstones both here and in Barkly consist 

 of fragments of altered lava and volcanic glass mixed 

 with grains of quartz, microcline felspar and zircon, 

 probably derived from the same source as the mater- 

 ials composing the Cave sandstone. Such beds as 

 these can be regarded as partly of ordinary detrital 

 and partly of volcanic origin ; they were undoubtedly 

 deposited under water, and thus support the evidence 

 already quoted to that effect. Tuffs play a very insigni- 

 ficant part in this formation, however. 



The basaltic lavas are dark greyish- or purplish- 

 brown in colour, and are usually evenly bedded. The 

 compact doleritic varieties sometimes show a rude 

 columnar structure and are less prone to alteration 

 than the amygdaloidal varieties. The latter weather 

 more readily and give rise to d^bris-coveied slopes on 

 the mountain sides rather than to kranzes. 



The Petrography of the Lavas. 1 



As a rule the lavas have the composition of basalts, 

 but in Barkly they are more nearly allied to the augite- 

 andesites, while from a few localities still less basic 

 varieties, enstatite-andesites, have been obtained. 



The mineral constituents are similar in both compact 

 and vesicular varieties, but the proportions in which 



1 G. C., vii., p. 65 ; and ix., p. 130. 

 15 



