306 ROCKS FROM SOUTHERN ABYSSINIA. [May I903. 



Total solid residue = 2'5806 grammes per litre. 



Per cent. 



Carbon-dioxide 1442 



Iron-peroxide 1'21 



Lime 1-21 



Magnesia 2*43 



Soda 42-18 



Sulphuric acid 5"18 



Silica 4-66 



Chlorine (by difference) . 28*71 



10000 



EXPLANATION OF PLATE XXI. 



Map of part of Southern Abyssinia, on the scale of about 70 miles to the inch. 



The topography is copied, by kind permission, from the map printed for use 

 at the meeting of the Royal Geographical Society on December 11th, 1899. 



The exact position and the area of the outcrops of the rocks can only be 

 roughly indicated, but the general succession of the localities is taken from the 

 dates on the labels accompanying the specimens. 



Discussion. 



The President spoke of the interest of the results worked out by 

 the Authoress, and of the clearness with which they had been laid 

 before the meeting by Prof. Bonney. Most of the rocks described 

 appeared to agree remarkably with the other African types usually 

 classed as Tertiary, but a few seemed to suggest those of the 

 Abyssinian plateau associated with the Jurassic limestones of that 

 region. It would be very interesting to know how the widespread 

 Tertiary igneous rocks of Africa were related to, or differed from, 

 those of the earlier geological epochs. 



Mr. Prior welcomed the paper as affording still further proof of 

 the wonderful uniformity in type of the more or less recent volcanic 

 rocks of the African continent. Members of the remarkable 

 alkali-rich rock-series, to which the eruptive rocks of the Great Rift- 

 Valley mainly belonged, were widely distributed over Africa from 

 the island of Pautelleria in the north to Madagascar in the south, 

 as well as in the Canary Islands, the Azores, and other islands off 

 the western coast. This constancy in type was almost sufficient to 

 constitute of Africa an immense petrographical province. 



Prof. Judd congratulated the Authoress on her discovery of the 

 wide range of rocks derived from an alkaline magma. Such rocks, 

 indeed, were now known to be widely distributed, and recent dis- 

 coveries in Australia and New Zealand had furnished additional 

 evidence on this point. 



Prof. Bonney, in reply to the President's question, said that he 

 believed that the district around and to the north of Addis Abeba 

 formed the southern end of the great Abyssinian plateau, but that no 

 evidence had been furnished as to the geological age of the basalts 

 of which this part was so largely composed. 



