Contributions to the Geology of British East Africa.—Part III. The Nepheline-Syenite and Camptonitic Dykes intrusive in the Coast Series

Abstract

I. Introduction. The interest of the great volcanic series of British East Africa is lessened by one drawback. The sequence of the lavas evidently extended over a prolonged period; a vast interval must have intervened between the eruptions that poured out the materials which form the rolling plains of Laikipia and the Athi, and those that built up the existing craters of Longonot and the Kyulu Chain. But this long series of volcanic rocks is not associated with contemporary sedimentary deposits whereof the age is known. The lacustrine beds interstratified with the volcanic rocks will no doubt some day yield up the vertebrate fossils which probably occur in them. Until then, however, the evidence for the correlation of the volcanic series of British East Africa remains deplorably scanty. Accordingly I was much interested when, as I was passing through Mombasa in 1893, Mr. C. W. Hobley told me of some dykes intrusive in Jurassic rocks at the back of Wasin, an island south of Mombasa, where the British East Africa Company then had a station. Mr. Hobley showed me a specimen of the rock forming Mount Jombo, which he regarded, no doubt correctly, as the massif whence the dykes were given off. As I was unable myself to visit the locality, Mr. Hobley most kindly procured for me specimens of the dykes, and gave me three specimens of the rock from Mount Jombo itself. This material forms the basis of the present paper. Mr. Hobley 1 has himself referred to the