Vol. 56.] GEOLOGY OP MOUNT KEN FA. 219 



Kenya had begun with the ejection of basic materials, and that the 

 phonolitic series was the more recent. This arrangement would 

 have been in accordance with the general rule of ejection in 

 order of decreasing basicity. In Bohemia, that classic land 

 of phonolites, the sequence of the Aquitanian (Upper Oligocene) 

 volcanic series, as determined by Hibsch, begins with nepheline- 

 and plagioclase-basalts, which are followed by tephrites, and these 

 in turn by phonolites, and finally come the trachytes. 1 



But further field-evidence refuted the idea that the basalts were 

 earlier than the more acid kenytes. The supposed tachylyte in the 

 agglomerates, in spite of its basic aspect both in hand-specimens 

 and under the microscope, proved to be of too low a specific gravity, 

 and must be regarded as a pitchstone ; and the field-evidence in the 

 Teleki Valley showed that the oli vine-basalt dykes are intrusive 

 into the kenyte-series, while the olivine-basalt flows must be 

 later, as they include some fragments of the kenytes. 



The oldest lavas found on Mount Kenya are the phonolites, flows 

 of which occur below the great kenyte-series of Mount Hohnel and 

 in the Teleki Valley. But there does not appear to be any sharp 

 separation in age between the phonolites and the kenytes, since 

 the former in the Mount Hohnel sequence are immediately covered by 

 the kenyte-tuffs, which are themselves traversed by phonolite-dykes. 

 These phonolite-dykes are darker and more basic than the phonolite- 

 flows, but the differences are slight. 



The phonolites and kenytes appear, therefore, to have overlapped, 

 and they probably represent two closely-allied types produced by 

 differentiation from the same rich soda-bearing magma. 



The last.stage in the volcanic history of the Teleki Valley quadrant 

 of Mount Kenya is represented by the olivine-basalts. The basalt- 

 and dolerite-dykes cut across the kenytes, and are in no place, so 

 far as I saw, cut by the nepheline-bearing dykes. The basalts do 

 not seem to reach the surface higher than the upper Alpine zone ; 

 Mr. Hobley found them on the edge of the southern forest-zone, at 

 the height of 8600 feet, and the basalts probably reached the surface 

 from a belt of secondary craters in the Alpine zone on the margins 

 of the kenyte-plug which choked up the original vent. Such a 

 centre of eruption probably occurred near Mount Hohnel, for flows of 

 fissile and columnar olivine-basalts occur around that peak. 



It cannot be affirmed, however, that the whole of the Kenya 

 basalts are later than the phonolite-kenyte series, for the fissile 



i See his ' Geologie ' (1885) p. 319, for the age of the series; for the sequence, 

 see his ' Ueber die Eruptionsfolge im bohmischen Mittelgebirge ' Sitzber. 

 deutsch. Naturw. Med. Ver. Bohm. Lotos (1897) No. 1. It is just possible that 

 the phonolite at the western foot of Mount Hohnel may be an intrusive sheet. 

 In the field I regarded it as a contemporaneous lava-flow, and only collected 

 one specimen from the middle ; but microscopically the rock has rather the 

 aspect of a dyke, and it is possible that the apparently vesicular upper surface 

 may have been due to the incorporation of tuffs. 



