CHAP. XII THE ARCHEAN ROCKS 227 



a few plants, ammonites, and reptile bones from the hills of 

 Mombasa, and a few sub-fossil shells of living species. The 

 only rocks to which any definite age was assigned were those 

 from which the above fossils came, and the gneiss, which was 

 regarded as Archean. 



In spite, however, of the absence of a definite series of 

 fossiliferous deposits, it is possible to determine the relative 

 ages of the rocks in East Africa, and to find among them re- 

 presentatives of most of the principal divisions of the geological 

 sequence. 



The earliest era of the earth's history is the Archean, which 

 is represented in Anglesey by a ridge of gneiss and by schists. 

 Rocks of the same kind underlie the whole of British East 

 Africa, and form two-thirds of its surface. Three days after 

 leaving Mombasa the traveller finds these rocks on the plains 

 near Taro, and continues upon them for 230 miles until past 

 Machakos. Here they sink beneath sheets of volcanic rocks, 

 from which they rise to the west, forming the whole of the 

 Uganda plateau and the Nandi Hills. 



The rocks of the Archean series may be divided into two 

 groups, the Upper and the Lower. The former occurs in a part 

 of the Iveti Mountains between Zuni and the pass of Kwazome, 

 and has been found by Mr. Scott Elliot on the flanks of 

 Ruwenzori. It is composed of a series of schists — banded 

 rocks which split into flakes like slates. These vary in colour 

 in accordance with the mineral which predominates in their 

 composition. Some are silvery gray, as they consist of mica 

 and quartz ; others are dark green, as they contain hornblende 

 instead of mica. The Lower Archean series consists of gneiss, 

 in which occurs bands of an irregular coarse-grained rock of 

 light colour (pegmatite), and of fine-grained fissile rocks which 

 are dark green or black. 



It is not easy to decide as to the exact origin of this 

 Archean series. The gneisses were probably formed from 

 various rocks, some of which were igneous and some sedi- 

 mentary. They have, however, all been completely altered 

 by burial deep below the surface, whereby they came under 

 the influence of the internal heat of the globe. They were 

 thus rendered crystalline, and their original characters obliter- 

 ated. The gneisses contain two sets of rocks, which were 



