222 THE GEOLOGY OF MOUNT KENYA. [May I9OO, 



EXPLANATION OF PLATES X, XI, & XII (pars). 

 Plate X. 



Geological sketch-map of the south-western quadrant of the Central Peak 

 and Upper Alpine Zone of Mount Kenya, on the scale of 1 inch to the mile. 



Plate XI. 



Microscope-sections of igneous rocks from Mount Kenya. 



Fig. 1. Kenyte of the plug (No. 499, p. 209). The section shows large corroded 

 anorthoclase-phenocrysts on the right, an altered olivine in the centre, 

 a rounded, broken crystal of pale green augite on the left, with one or 

 two prisms of apatite and dark patches of segyrine-grains. X 12. 



2. Kenyte with tacbylytic base (No. 519, p. 213). The large corroded 



crystal at the top is anorthoclase ; the rounded crystal below it is 

 yellow olivine. X 10. 



3. Kenyte-lava (No. 508, p. 212). The dense base is crowded with 



jegyrine-grains and minute felspar-microliths showing flow- structure : 

 throughout the slide are rounded patches of a coarser- grained aggre- 

 gate of felspar-laths and a?gyrine : the felspar-phenocrysts are small 

 (one is shown in the section to the left of the central patch), and in 

 many cases have suffered almost total reabsorption. X 12. 



4. Nepheline-syenite of the Central Core (No. 496, p. 208). In the centre 



of the section is an irregular crystal of barkevicitic hornblende: the 

 colourless mineral with a dark inclusion above the hornblende is 

 nepl eline, forming part of a mosaic of felspars and nepheline, finer- 

 grained than the rest of the section. Below the hornblende-phenocryst 

 are large felspars in Carlsbad twins. On the left, and surrounded by 

 opacite, are crystals of pale purplish augite fringed by a zone of 

 aegyrine. X 20. 



5. Phonolite dyke-rock (No. 456, p. 210). The dark phenocrysts are 



a?gyrine, which also occurs in needles interspersed between the felspar- 

 laths and in dense patches round the clear hexagonal and rectangular 

 sections of nepheline. X 20. 



6. Glassy basalt forming a composite dyke with phonolite (No. 457, p. 210). 



The section shows clear, sharply-defined laths of labraclorite, and 

 numerous small rhombic and hexagonal sections of olivine, with a few 

 augite-prisms, in a dense glassy base crowded with magnetite-grains. 

 X20. 



Plate XII (pars). 



Fig. 1. Phonolite-flow (No. 490, p. 215) The section shows the same mineral 

 constituents as PI. XI, n'g. 5, but the felspar-laths of the base do not 

 exhibit any well-marked flow-structure. X 12. 

 2. Kenyte from Mount Hohnel (No. 464, p. 217). The section shows the 

 least glassy specimen of kenyte, as seen between crossed nicols. The 

 large phenocrysts exhibiting very fine twin stria tions are anorthoclase. 

 The clear-cut octagonal section showing a nearly rectangular cleavage 

 is almost colourless augite (diopsidej. The small, sharply-defined 

 hexagonal sections are apatite. The base, which is rather dense with 

 opacite-grains, contains numerous lath-shaped felspars. X 12. 



