CH. ix THE SARAWANGA PLAINS 129 



are displayed, often much decomposed and developing a spheroidal 

 character, or lying in large blocks all around. Overlying the 

 basaltic rocks in various localities occur foraminiferous clays and 

 other submarine deposits. This great region of plains is partially 

 divided into two by the projecting mass of the dacitic district of 

 Ndrandramea, the slopes of which descend to within 3 or 4 miles 

 of the coast between the Sarawanga and Ndreketi rivers. For 

 convenience of description I will deal with these two sub-regions 

 separately under the names of the Sarawanga and Ndreketi plains. 



THE BASALTIC PLAINS OF SARAWANGA. These plains extend 

 about 6 miles inland to the village of Tembe-ni-ndio on the head- 

 waters of the Sarawanga river. The prevailing type of basalt in 

 this region is a porphyritic divine-basalt showing a few large 

 crystals of glassy plagioclase and having a specific gravity of 2*84 

 to 2*9. They are neither vesicular nor scoriaceous and are referred 

 to genera 25 and 37 of the olivine class. The felspar-lathes of the 

 groundmass average '2 mm. in length, and there is a little 

 interstitial glass. They cannot often be distinguished in their 

 characters from the olivine-basalt displayed in vertical columns, 4 

 to 5 feet in diameter, on the lower slopes of Seatura at the back of 

 Tembe-ni-ndio (page 63). It is highly probable that most of the 

 basalts of these plains belong to lava-flows that descended from 

 the great Seatura vent. In the lowlands it is much decomposed, 

 and a spheroidal structure is frequently developed during the 

 disintegrating process, just as has been noticed in the case of the 

 Mbua and Ndama plains on the west side of Seatura. The 

 rounded blocks that commonly occur on the surface may be 

 regarded in each instance as the nucleus of a weathering spheroidal 

 mass. When this rock is exposed unaltered in the streams it is 

 usually massive or non-columnar. 



There is a less common type of basalt in this region which 

 perhaps may represent the upper portion of these basaltic flows. 

 I found it exposed in the bed of the Selesele river about half-way 

 between Lekutu and Sarawanga and about 2 miles inland, where it 

 formed vertical columns I \ feet across. It differs principally in the 

 presence of a few small amygdules and in the greater amount of 

 interstitial glass. The columnar basalt that Dana in the " Geology 

 of the United States Exploring Expedition " describes as occurring 

 at the mouth of the same river probably belongs to the same flow. 

 He remarks that a few hundred yards back from the " Watering- 

 place " there is an exposure of columnar basalt, the columns being 

 vertical, i to 2\ feet in diameter, and usually six-sided. 



K 



