﻿part 3] 



VOLCANTC ROCKS OF MOZAMBIQUE. 



239 



• 



• 



A. 



B. 



C. 



D. 



E. 



F. 



SiO, 



56-34 



54-94 



58-37 



57-81 



58-62 



61-49 



Al ,0, 



17-86 



19-34 



16-65 



18-74 



21-50 



18-25 



I'e..O :) 



4-43 



1-80 



4-09 



5-76 



0-47 



1-77 



FeO 



3-81 



4-52 



3-03 



0-42 



3-65 



3-13 



MgO 



0-52 



1-11 



0-37 



tr. 



0-56 



0-41 



CaO 



1-97 



2-05 



1-66 



1-28 



0-88 



1-65 



Na 2 0 



7-33 



8-39 



7-28 



9-35 



7-95 



6-78 



K,0 



4*72 



5'93 



5*46 



4'52 



5'47 



5*47 



H„0 



2-04 



0-32 



2-36 



1-50 



1-12 



0-26 



Tib, 



0-67 



0-67 



0-21 





0-06 



0-51 



CO.," 



0-71 













P.,0.5 



tr. 



0-18 



0-08 







0-09 



MnO 



0-28 



tr. 



0-43 . 



■ tr. 







Totals 



100-68 



99-90 1 



99-99 



99-38 



100-28 



99-81 









1 Including- ZrO 2 = 0-38, and SO., = 0-27 per cent. 



A. Phonolite. Near the Sanhuti River, Mozambique (an. Holmes). 



B. Glassy katoforite-phonolite. W. Kibo, B.E. Africa (an. Eyme). 



C. Phonolite. Base of Mount Hohnel, Mount Kenya, B.E. Africa (an. Prior). 



D. Tinguaite. Edda Gijorgis, Abyssinia (an. Prior). 



E. Phonolite. Nosy Komba, Madagascar (an. Pisani). 



F. Phonolitic trachyte. Ravin des Fleurs Jaunes, Reunion (an. Boiteau). 



XI. Tephmtic Pumice. 



Among the detritus that litters the northern bank of the 

 Monapo, near its entry into Mokambo Bay, numerous fragments 

 of dark-grey pumice occur. Unfortunately the rock has nowhere 

 been seen in situ, although it may be traced for a few miles along 

 the northern bank of the river. It must, therefore, have come 

 from the district between the Monapo and Murimatigri — an area 

 originally covered by flows of amygdaloid, but now greatly dissected 

 by erosion. 



In chemical composition, the rock differs strikingly from the 

 basalts and andesites of the neighbourhood, whereas it appears to 

 be closely related to the alkali and associated lavas of the Sanhuti- 

 Biver district. 



Under the microscope, the rock is seen to be made up of a 

 dense brown glass containing in places irregular patches of less 

 intense colour, in which crystallization had just made a be- 

 ginning. Within these local clearings, very minute crystals of 

 magnetite or ilmenite and micro lites of felspar can just be 

 seen under a quarter-inch objective. The felspar shows no trace 

 of twinning, but a maximum extinction-angle of 25° points to its 

 identity as andesine. This result suggested that the rock might 

 be an andesite-pumice. Against this view, however, was the fact 

 that the radium-content of the lava was 2-59x10" 13 grms. per 

 grm., a 'figure which would be surprisingly large for a rock of 

 andesitic composition. In these circumstances it was thought 



