DERIVATION OF THE FAUNA 19 



stones and grits are the oldest sedimentary deposits and 

 have been assigned to Triassic age. The dip of all of these 

 beds is toward the coast as is that of the Archaean gneiss 

 which underlies the Taru grits and conies to the surface 

 near Mackinnon Road station. The narrow coast belt of 

 sedimentary rocks is a very incomplete geological series 

 without any formations of Tertiary age or any older than 

 a doubtful Triassic sandstone. No vertebrate remains 

 have been found in any of these formations. 



From the gneissic rocks have been derived the bright- 

 red gritty soil of universal occurrence in the desert nyika. 

 In the grassy uplands a black cotton soil is met with on 

 grassy steppes, notably the Kapiti and Athi Plains. Its 

 dark color is apparently due to vegetable humus and alka- 

 line salts from the red soil. The volcanic lavas are made up 

 chiefly of trachytes on the higher altitudes, with phonolites 

 underlying them, which are found exposed at the lower 

 levels near the Archaean rocks. The lavas give rise to a 

 bright-red clayey soil containing a large per cent of ferric 

 oxide and resembling in color the red gneissic soil of the 

 desert lowlands. So wide-spread are these various soil con- 

 ditions over areas extending through several faunal zones 

 that no botanical or zoological significance in distribution 

 has been found attributal to their influence. 



B — Derivation 



In all the literature pertaining to zoology there is noth- 

 ing, perhaps, more unstable than the theories of derivation 

 based upon paleontological evidence. The discoveries of 

 new forms of animals or groups in new horizons and locali- 

 ties which are constantly taking place give a kaleidoscopic 



