228 THE GEOLOGY OF BRITISH EAST AFRICA part iii 



probably igneous in origin, and were forced into the gneiss in a 

 molten condition. When the main series was altered, these 

 igneous intrusions underwent the same change. One set con- 

 sisted of so-called "acid" rocks, as they contained much 

 quartz ; these have formed " pegmatite " veins. The other set 

 was poor in quartz, but rich in such materials as iron and 

 magnesia, and thus had a more complex chemical composition ; 

 they therefore underwent more marked alterations, and were 

 crushed into a series of dark -coloured banded schists in the 

 way so clearly described by Teall in the case of the Scourie 

 Dyke in Sutherland. These rocks, therefore, carry us back to 

 a very early period in the earth's history, for they doubtless 

 date from the Archean era, and are of approximately the same 

 age as the gneiss of Anglesey. Though their interpretation is 

 difficult, they tell us that Africa was originally a country where 

 igneous activity prevailed, and they remind us that all the 

 rocks, now exposed on the surface, were once deeply buried 

 underground. 



After leaving the Archean system there is a gap in the 

 geological record, and we have no certain knowledge of the 

 condition of Equatorial Africa during the first four of the five 

 ages of the era characterised by the " ancient forms of life." 

 Deposits formed during this interval are known, such as the 

 Karagwe series, to the west of the Victoria Nyanza, and the 

 Lualaba series of the Upper Congo basin. But they have not 

 yet yielded any fossils ; and as the shales and sandstones of 

 which they consisted have been crushed into slates or consoli- 

 dated into quartzites, they give no certain knowledge of the 

 condition of the country at the time of their formation. 



The record does not begin again till the upper part of the 

 Carboniferous period, when England was covered by the swamps 

 and jungles, in which grew the vegetation that has formed our 

 coal-fields. 



In the hills to the west of Mombasa there are some sand- 

 stones in which occur the famous water - holes of Taro. 

 Thornton found a few plant impressions in these beds, and on 

 this evidence the sandstones have been assigned to the Carbon- 

 iferous. However, in the Sabaki valley I obtained more 

 definite evidence of the Carboniferous age of some of the 

 deposits, by finding a series of fossils containing some fish 



