The Ruri hills in western Kenya represent an area of Miocene volcanic activity with development of carbonatite ring dikes. Associated plutonic rocks are alkaline syenites and a syenodiorite, which occur as dikes and plugs and were probably formed at the same time as the ijolites at Usaki, three miles to the northeast. The locations, petrography, petrogenesis, and paragenesis are given, and it is hypothesized that the present composition of the syenites is the result of replacement, for which there is good evidence.

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