274 MR. A. J. C. MOLYJSTEFX ON THE [May I903, 



is situated upon the denuded scorise and lava-flows of an extinct 

 crater, and in the trenches these ejectamenta can he easily studied. 



This area of past volcanic action may therefore well be known 

 as the Tuli Lavas. In the traverse described, which crosses the 

 scoriaceous deposits 40 miles to the east of Tuli, these volcanic 

 rocks form a belt 30 miles wide. On the southern boundary they 

 gradually die out against a series of parallel ridges of fine sand- 

 stone, which have been crushed and minutely cross-fissured, the 

 reticuate fissures being now infilled with silica. The rock is there- 

 fore extremely hard, and has weathered in rugged parallel ranges, 

 all following a general north-easterly direction. 



It can be seen that this crushing and shearing has been caused 

 by intrusive dykes. In the Chilichukwe River, near an old fortified 

 hill 8 miles east of the Umsingwane River, the gradually - 

 extending influence of a dyke is very noticeable. The rock is there 

 a fine white felspathic sandstone, with few main fissures, but cracks 

 begin 30 feet; away, and become more frequent until they are only 

 an inch or less apart. The sandstone then becomes altered to 

 quartzite, and the dyke breaks in. It shows tabular jointing at the 

 sides, where it resembles a stack of tiles on edge, 1^- to 2 inches 

 wide ; farther in the joints are 6 inches apart, while the centre is 

 irregularly jointed — the total breadth being 12 feet. 



This alteration by contact-metamorphism is noted as a cause for 

 the induration of some of the long ridges of veined sandstone which 

 extend across this portion of the Tuli district, while others are due 

 to cracking along faults and maybe almost called crush-breccia. 

 AVhere this induration occurs, the hardened rock has resisted decay t 

 forming ridges and lines of hills rising out of the generally level 

 country of the Tuli district, such as the bluff at Massabi's on the 

 Limpopo River, and the Samkoto Cliff on the Umsingwane River. 

 Hence the provisional term of Samkoto Series is applied to the 

 soft sandstones and the indurated veined quartzites of this locality. 



South of Samkoto Cliff, they lie unconformably on garnetiferous 

 quartzites, foliated schists, hornblendic gneiss, etc., highly inclined 

 to the north — and farther south these schists merge into gneiss and 

 granite, containing veins of pegmatite, and quartz with mica- 

 crystals several inches across. 



Lying unconformably in a basin in the Samkoto Series are the 

 generally-horizontal beds aud workable seams forming the Umsin- 

 gwane Coalfield. These softer sediments are capped by conglomerate, 

 and extend from the Umsingwane River to the Singwisi, 5 miles 

 away, where there are many intrusions of dolerite, which have 

 altered the coal-seams at that end of the basin into a semi- 

 anthracite. The coal- bearing beds also crop out in the Limpopo 

 Valley, where they rest upon the tilted metamorphic rocks. The 

 Tuli Lavas are later than, and overlie, the coal-bearing beds in this 

 district. 



The southern sediments, except for a calamite found in the 

 Umsingwane Coalfield, have so far yielded no fossils, but in 



