TRANSACTIONS OF SECTION C. 759 



have broken out, and it is the interception of the drainage by the material thrown 

 out from these vents that forms the lakes Naivasha, Nakuru, and Elmeteita. 



Highly acid and ultra-basic rocks are represented. Kilimanjaro and the 

 Kyulu Mountains are chiefly built up of basic rocks, while the lavas of Lykipia 

 and the Mau plateaux are chiefly acid. It appears that the latter localities have 

 been the seat from which acid lavas have continued to be poured from times prior 

 to the first eruptions of Kilimanjaro up to the present day. 



The basic lavas of Kilimanjaro do not extend very far from the original point 

 ■of issue. .A.t least this is so to the north, for no lavas were found on the plains of 

 Lytokitok, distant thirty miles north of Kilimanjaro. On the other hand the 

 acid lavas of Mau and Lykipia extend for great distances. Eastwards they 

 stretch as far as the Athe plain, about fifty miles, and westwards to near the shores 

 of Victoria Nyanza, a distance of nearly one hundred miles. 



Further westward, in Busoga and Buganda, basic igneous rocks pierce the 

 metamorphic roclcs, but without possessing any general trend. 



With the exception of the still active volcanoes it is impossible to state even 

 the approximate geological age of any of the eruptions. Some of the volcanoes are 

 possibly only dormant, others are certainly extinct, but none appears to be of great 

 geological antiquity. All that can be safely asserted is that they are long 

 subsequent to the deposition of strata containing ammonites, for, whereas the 

 conglomerates of these sedimentary deposits contain pebbles of schist and gneiss, 

 they nowhere yield fragments of igneous or volcanic rocks. 



5. Eeport on the Volcanic Phenomena of Vesuvius. — See Eeporfcs, p. 471. 



10. On Quart:: Enclosures in Lavas of S'romhoU and Stromholicchio, and 

 their Effect on the Composition of the Eoch. By Professor H. J. 

 Johnston-Lavis, M.D., M.B.G.S., B.-es-Sc, F.O.S. 



In a recent dolerite lava stream that reaches the sea at Punta Pietrazza, on the 

 island of Stromboli, are numerous inclusions of vein quartz and quartzite. These 

 attain several centimetres in diameter ; some specimens are almost clear glassy, 

 while others are opaque and granular. 



They have undergone more or less softening and fluxion, if not actual fusion, 

 by the lava. They are surrounded by a glassy crust containing numerous augite 

 -crystals, more especially at the periphery. AVhere the glassy envelope has formed 

 veins penetrating along the fissures in the quartz, augite crystals have crystallised 

 out of this vitreous fluid. The amount of augite in the vicinity of these quartz 

 •enclosures is greater than the average in the surrounding lava, showing that the 

 quartz has afforded a material necessary for the individualisation of augite. The 

 crystallisation of such out of the glass envelope would have been more complete 

 if sudden cooling of the lava had not prevented such a result. 



The small island of Stromholicchio, close to Stromboli, is the wreck of an old 

 volcanic neck. The rock composing it is lighter than the lavas of Stromboli, being 

 of a purple tint, in which dark bottle-coloured and also bright emerald green 

 augites are visible, the latter being fewer but very striking. The Strombolicchio 

 rock is crowded with quartz enclosures, more opaque, more granular, and en- 

 wreathed with numerous emerald green augites. This green crust is seen micro- 

 scopically to be composed of mixed grains of quartz and augite. We can trace 

 the emerald green augites to an origin in the quartz which has combined with the 

 Tesidual fluid of basic oxides with insufficient silica for the individualisation of a 

 mineral to form an augite. 



The process is seen better here on account of the slow cooling of the plug and 

 the absence of the mechanical disturbance in the flowing stream of lava of the 

 Punta Pietrazza. 



I have elsewhere shown the olivine nodules and many loose crystals are nothing 

 more than altered limestone enclo=!ures, and here we see quartz adding augite to a 

 lav a which may owe its diminished acidity in part to the absorption and conversion 



