BY H. I. JENSEN. 547 



the Kenya region) at the same time as the development of these 

 trough-faults. 



From an Australian point of view, one of the most important 

 alkaline areas in the world is that of Abyssinia. The funda- 

 mental rocks are, according to Blanford,* the Archaean and 

 Metamorphic. Between the metamorphic rocks and the traps 

 (which designation includes the alkaline lavas) there is found a 

 series of sandstones and limestones, the latter of which contain 

 Jurassic fossils. Dr. Schimper, the collector of a large number 

 of specimens of Abyssinian traps now in the British Museum, 

 describes them as volcanic. Prior, apparently on petrological 

 grounds alone, classes theui as hypabyssal. His position seems 

 to me unjustifiable, inasmuch as the collector's field-observations 

 are more important in deciding this question than minute 

 structure worked out in the laboratory. Further, Blanford's 

 sketches {/oc. clt.) show a physiographic type resembling that met 

 with in the Warrumbungle Mountains, New South Wales, viz., 

 a development of flat-topped mesas capped with lava and buttes 

 of igneous rock. My experience of alkaline rocks shows that the 

 lavas frequently possess the same petrological characters as the 

 corresponding fine-grained dyke and sill rocks. 



The rocks examined by Prior are remarkably like the Austra- 

 lian riebeckite trachytes, phonolites, solvsbergites, etc., both in 

 microscopic structures and chemical characters.! They include 

 rocks classed as grorudite, paisanite, tinguaite, trachyte, solvs- 

 bergite and basalt. The sequence seems to be the same as 

 observed in Australia, the basic rocks being later than the 

 alkaline ones. The age of the flows is not certain, but they 

 appear to be certainly Post-Jurassic, and as similar rocks of 

 Eocene age are known in Somaliland whose geological structure 

 is analogous to that of Abyssinia, it appears probable that the 

 Abyssinian foyaitic lavas are also Tertiary. 



* Geology and Zoology of Abyssinia, 

 t See G. T. Prior, Min. Mag. Vol. xii. p. 255, and compare with my 

 Papers on Australian areas cited elsewhere. 



