Vol. 56.] NEPHELINE-SYENITE AND CAMPTONITIC DYKES. 223 



11. Contributions to the Geology of British East Africa. — Part 

 III. The JNepheline-Syenite and Camptonitic Dykes intru- 

 sive in the Coast Series. By Prof. J. W. Gregory, D.Sc, 

 F.G.S. (Read January 21th, 1900.) 



[Plate XII, pars.] 



Contents. 



Page 



I. Introduction 223 



II. The Nepheline-Syenite of Mount Jombo 223 



III. The Camptonitic 'Dykes 224 



IV. The Age of the Jombo Series and the Duruma Sand- 



stones 225 



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 inter- 

 vened 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 contem- 

 porary sedimentary deposits whereof the age is known. The lacus- 

 trine 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 intru- 

 sive 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 occurrence of these rocks 

 and marked the exact locality of one of the dykes on a map accom- 

 panying a paper published in 1895. 



II. The Nepheline-Syenite of Mount Jombo. 



Mount Jombo or Jomvu is situated in long. 39° 13' east and in 

 lat. 4° 26' south 2 in the country of the Wadigo, in the south-eastern 



1 ' Upon a Visit to Tsavo & the Taita Highlands ' Greogr. Journ. vol. v 

 (1895) pp. 560-61 & map, p. 559. 



2 Admiralty Chart, No. 1390. 



e2 



