IV MBUA AND NDAMA PLAINS 57 



of the series of stagnant pools in the bed of the stream is stained 

 blood-red by iron-oxide, a circumstance that has naturally given 

 rise to native legends of a corresponding hue. These fragments of 

 iron ore, which lie between 100 and 150 feet above the sea, 

 represent the final stage of a process which is now no doubt in 

 operation on the bottom of the neighbouring pools and small 

 swamps. Their presence on the surface goes to indicate that this 

 open country has been for ages a land-surface free from forest, as it 

 is in our own time. 



In a similar manner, the extensive disintegration of the basaltic 

 rocks that form these plains affords evidence of the great antiquity 

 of these "talasinga" plains in their present unforested condition. 

 The extent to which these rocks have weathered downwards is very 

 remarkable. Between Ndama and Mbua they are decomposed to 

 a depth often of eight or ten feet below the surface. This is well 

 exhibited in the sides of deep channels excavated by the torrents 

 during the rains. Here the spheroidal structure is well brought out 

 in the disintegrating mass, all stages being displayed in the form- 

 ation of the boulders that are scattered all over these plains. 



In one locality, near the lower course of the Ndama river, a 

 thickness of 25 feet of decomposed rock was exposed in a cliff-face. 

 In this case the rock was a porphyritic basaltic andesite, the 

 disintegrating process having affected the whole thickness of the 

 large spheroidal masses with the exception of a hard central 

 nucleus of the size of the fist. In one of these nuclei by my side it 

 is apparent that during the extension of the weathering process the 

 phenocrysts of glassy plagioclase become opaque long before the 

 groundmass is affected. In this specimen the stage of disintegra- 

 tion as affecting the felspar phenocrysts is at least one and a half 

 inches in advance of that affecting the groundmass. 



This great disintegration of the basaltic rocks, which as pointed 

 out on page 64 is also in progress on the slopes of the adjacent 

 spurs of Mount Seatura, is more characteristic of the porphyritic 

 basaltic andesites than of the olivine-basalts. It is to the spheroidal 

 weathering that we must look for an explanation of the rounded 

 boulders so frequent in these districts. It may also be inferred 

 that the soil produced from this extensive decomposition of the 

 rocks is often very deep. At the Wesleyan Mission Station at 

 Mbua, on level ground nearly a hundred feet above the river, a well 

 has been sunk to a depth of 20 feet in soil of this description ; and 

 away to the westward a similar thickness of soil produced by the 

 same cause is in places to be observed. 



