546 THE DISTRIBUTION, ETC., OF ALKALINE ROCKS, 



folding is proved by the horizontality of the Karroo strata (Permo- 

 Carboniferous), and of later sediments such as the Uitenhage 

 (Cenomanian). The latter were deposited in erosion-valleys in 

 the Karroo in the period of the Cenomanian transgression. In 

 Madagascar similar conditions obtain. The Drakenberg Moun- 

 tains consist of Tertiary eruptives, including trachytes, andesites, 

 and probably alkaline rocks also, which have been erupted along 

 a fissure-line. The intrusion of laccolites of the Cedar-tree type 

 has slightly tilted the Karroo sediments to the west. 



In the north of Madagascar, also a horst or relic of Indo- 

 Africa, alkaline lavas occupy a considerable area, and are, as in 

 Bohemia, Sumatra, Central France, and Brazil, practically super- 

 imposed on the massive. They are all of early or middle Tertiary 

 age, and have issued, as at Hogbau in Wiirtemberg, along radial 

 and peripheral cracks. 



The alkaline lavas of Kenya and Kilimanjaro in East Africa 

 have been poured out along the immense faults which border the 

 Great Rift Valley of Africa on the east. The volcanoes of Kenya 

 and Kilimanjaro are practically situated in a trough which runs 

 parallel to the Rift Valley, and constitutes a continuation of 

 the Lake Nyassa Rift (see Suess). This area is therefore a por- 

 tion of Africa which is being broken up by intense faulting. 



East of Lake Tanganyika similar lavas occur overlying hori- 

 zontal Carboniferous rocks, but they are nevertheless of Tertiary 

 age. 



From Somaliland riebeckite trachytes and phonolites have 

 been recorded, and are considered to have been erupted in the 

 Eocene. These rocks are found along a rift running parallel to 

 the Rift Valley. The horizontal Mesozoic sediments, deposited 

 at the times of the Jurassic and Cenomanian transgressions, 

 which overlie sometimes almost horizontal Carboniferous rocks 

 and sometimes very old Palaeozoic and Archaean rocks in East 

 Africa, Arabia and Socotra almost at the same level, prove that 

 these three lands were continuous during the Mesozoic, and have 

 been severed in the Tertiary period by enormous trough-faults. 

 The alkaline rocks have been produced (and are still produced in 



