Vol. 56.] AND CAMPTONTTIC DYKES IN BRITISH EAST AFRICA. 



225 



the titaniferous augites, so striking a feature of the closely-allied 

 monchiquites from Fernando JNoronha described by Mr. G. T. Prior. 1 

 The analcite of the groundmass is small in amount. (See PI. XII, 



fi °- ^ ...... 



A slide cut from a second specimen exhibits a fine-grained 



segregation-nodule, while the groundmass of the rock is coarser in 



grain, and contains more recognizable crystals of analcite. 2 I 



could find no quite satisfactory case of symmetrical extinction in 



the plagioclase, but the best case gave extinctions of 11° and 9°, 



which makes it probable that the species is oligoclase. The specific 



gravity of this rock is 2-59. (See PI. XII, fig. 5.) 



This rock presents characters intermediate between felspathic 



camptonites and the olivineless variety of monchiquites, for which 



the late J. F. Williams proposed the name f our chit es. 3 From 



the rocks of the typical locality they differ mainly in containing a 



fairly large amount of felspar ; the minerals appear to be fresher, 



while the augites are seldom twinned, have no violet pleochroic 



borders, and are very much smaller. 



IV. The Age of the Jombo Series and the Duruma Sandstones. 



The main interest of this small igneous series is that it 

 provides the means of fixing a maximum date for the eruption 

 of the easternmost of the volcanic rocks of British East Africa. 

 Unfortunately, however, there is some doubt as to the exact age 

 of the rocks into which the camptonites are intrusive. 



The general geology of the belt of coast-lands between the Uganda 

 road west of Mombasa on the north, and the Anglo-German 

 boundary west of Wanga on the south, has been briefly described 

 by Mr. C. W. Hobley. 4 He shows that the country between the 

 Archaean plateau and the sea consists of three belts of sedimentary 

 rocks ; along the shore are Pleistocene sands, raised coral-reefs, and 

 limestones ; behind these are some fossiliferous shales with Jurassic 

 fossils ; and to the west of this series is a belt of massive sandstones, 

 which form the range of the Shimba Mountains. The volcanic 

 rocks are intrusive into these massive sandstones, the age of which 

 it is necessary to consider. 



The Jurassic rocks of Eastern Equatorial Africa were first dis- 

 covered by J. M. Hildebrandt 5 in 1876, near Mombasa, where he 

 collected some fossils described by Beyrich. 6 The Jurassic rocks of 



1 ' Note on the Occurrence of Eocks allied to Monchiquite in the Island of 

 Fernando INoronha' Min. Mag. vol. xi (1897) pp. 171-75. 



2 L. V. Pirsson, ' On the Monchiquite or Acalcite Group of Igneous Eocks' 

 Journ. Geol. vol. iv (1896; pp. 679-90. 



3 ' The Igneous Kocks of Arkansas ' Ann. Rep. Geol. Surv. Arkans. 1890, 

 vol. ii, p. 107. 



i Geogr. Journ. vol. v (1805) pp. 560-61. 



5 ' Von Monabassa nach Kitui ' Zeitschr. Gesellsch. Erdk. Berlin, vol. xiv 

 (1879) p. 241. 



6 ' Ueber Hildebrandt's geologische Sammlungen von Monabassa ' Monatsber. 

 k. Preuss. Akad. Wissensch. Berlin (1878) pp. 767-75. 



