v MOUNT SEATURA 69 



one's self in a region of limestone. The degree of alteration varies 

 considerably, those most altered being light-coloured and greenish, 

 whilst the others are darker, the specific gravity ranging from 2^69 

 to 279. In spite of these differences almost all of them appear to 

 belong to the same eruptive series, being as a rule sharply dis- 

 tinguished from the prevailing unaltered surface basaltic rocks of 

 the slopes of Seatura by the size of the felspars of the groundmass, 

 which average about "3 mm. in length, whilst those of the basaltic 

 rocks just alluded to average only '17 or 'i8 mm. long. These 

 rocks are also well displayed in the sides of the Nandi Gorge ; and 

 from their mode of exposure by river-erosion, as well as from their 

 relatively coarse crystalline texture, and from their alteration, it 

 may be inferred that they are older and more deeply situated than 

 any of the Seatura rocks before referred to. Whether these rocks, 

 which extend over an area of some square miles, have been altered 

 by solfataric action or contact-metamorphism, 1 I will not now say. 

 The fact remains, however, that they are best exposed wherever 

 the streams have worn deeply into the floor, and lower slopes of 

 the great basin, or have cut down into the mountain-mass as in the 

 case of the Nandi Gorge. The rocks that lie in loose blocks on the 

 surface either at the bottom of the basin or on its slopes extending 

 even to the very summit of the mountain (see page 67), are 

 characteristic blackish olivine-basalts of the type prevailing around 

 the mountain's slopes. These propylites are most frequently 

 exposed as dykes in the beds of the rivers at the bottom of the 

 basin. Such dykes vary from 4 to 6 feet in thickness, and they 

 are very conspicuous when they stretch across the river's breadth 

 projecting more or less above the water. From their frequency it 

 may be inferred that in many other small exposures, ill suited for 

 displaying the mode of occurrence of the rock, we have also to deal 

 with dykes. Judging from four dykes that were particularly 

 examined, they are all vertical or nearly so, and all run in much the 

 same direction, namely, N.N.W. S.S.E. or N.W. S.E., whether on 

 the north or south side of the great basin. In one instance, a rudely 

 columnar structure across the thickness of the dyke was observed. 

 From their exposure in river-beds it was rarely possible to ascertain 

 much more than is given above. However, in the bed of a river, a 

 mile above Ndriti, there was an extensive exposure of a highly 



1 This question, which has so often been raised with respect to the propylites, 

 will probably receive a different answer from different localities. The matter is 

 further discussed on later pages. 



