Vol. 70.] MIOCENE OF THE VICTORIA MTANZA, ETC. 145 



jasper, and andesite in a dark-green fine-grained matrix, so as to 

 present the appearance of a puddingstone. The contained rocks do 

 not, however, weather unequally, as might be expected, but in 

 perfect uniformity with the matrix — -as if the rock had been cut 

 through sharply and cleanly with a knife, irrespective of its varied 

 contents. Microscopic examination suggests that the diorite is 

 merely the deep-seated form of the augite-andesite, the augite in 

 both cases being uralitized ; and the uniformity of weathering is 

 further accounted for by the intimate manner in which both the 

 matrix and the contained rock-fragments are penetrated by a 

 network of thin veins of quartz. 



Crags of this agglomerate occur on both sides of the Ogo Valley 

 and up the Dodo Valle} 7- to Dodo and Taygoreh, as well as south 

 of Metamala. On the periphery of the agglomerates, for instance, 

 at Moroya, more than 5 miles south-west of Metamala, a dark- 

 green banded tuff occurs (see footnote), very hard and compact, also 

 silicified, but without the big rounded fragments ; and this banded 

 tuff also builds up the hills of Godateli and Yangena west of 

 Metamala. The summit of Godateli is traversed by a thick vein 

 of white quartz striking north-north-east and south-south-west, 

 which has so resisted weathering as to cause the formation of this 

 rounded hill above the rolling plain of tuff. The low hill between 

 the Metamala and Moroya plains (covered by quartz-ironstone 

 breccia) similarly owes its formation to the presence of quartz- 

 veins and their resistance to weathering. Although time did not 

 allow me to determine the limits of the agglomerate and tuff on the 

 south-east, it is clear that they cover an area exceeding 5 miles in 

 diameter, and mark the centre of eruption of the augite-andesites, 

 which probably extend as far south-eastwards beyond the agglome- 

 rate as they do westwards and north-westwards. 



The agglomerate and tuff weather to a mixture of ironstone and 

 quartz, and honeycombed patches of murrain are frequent here, 

 as on other ferruginous soils. 



the orthoclase, -which is subordinate in amount and turbid. Green horn- 

 blende is abundant in imperfect crystals (up to 3 mm.) with a uralitic 

 aspect, the uralite changing to chlorite, calcite, and epidote. Some biotite 

 (associated with the hornblende) was originally present, enclosing apatite, but 

 is now altered to epidote and calcite. Sphene and magnetite are accessory. 

 Quartz is abundant in interstitial granules, sometimes with strings of fluid 

 inclusions. Fine veins traverse the slide, filled with quartz and epidote, or 

 else recemented by secondary minerals — a fractured quartz-grain by secondary 

 quartz, felspar by felspar, and hornblende by hornblende. 



In the tuff of Moroya, on the southern periphery of this outcrop of agglo- 

 merate, the fragments consist essentially of the constituents of the diorite, 

 but with diopside only partly uralitized, the uralite changing into chlorite 

 (exactly as in the andesite of Kodondo). Hence the hornblende of the diorite 

 is probably all derived from augite, and the original rock would have been a 

 quartz-augite-diorite. 



The tuff, both at Metamala and at Moroya, is a crystal-tuff composed of 

 the constituents of the diorite, and numerous fragments of microlitic felts of 

 quartz with oligoclase-microlites ; both rocks are traversed by fine veins of 

 quartz (sometimes with epidote), and are evidently very siliceous. 



