XXIII ALTERED VOLCANIC MUD-ROCKS 325 



displayed on the flanks of mountain-ranges buried beneath basic 

 tuffs and agglomerates, as in the case of Mount Mariko (page 187) 

 and of the mountain-slope behind Mbale-mbale (page 158); or 

 they may be found at much lower levels as at Savarekareka Bay 

 (page 190), where, however, they assume sometimes a peculiar 

 character. Though in the last-named locality the alteration is 

 possibly connected with thermal metamorphism, it is probable 

 that in most instances it is a normal interstitial change occurring 

 in beds of some antiquity which are covered over by a considerable 

 thickness of later deposits. In places, where these rocks have 

 been subjected to much hydration in the weathering process, they 

 become red in colour, as is found on the flanks of Mount Mariko. 



Samples of the Altered Volcanic Mud-rocks 



A. From between 400 and 500 feet above the sea on the south 

 slope of the Mariko Range. . . . The characters and mode of 

 occurrence of this rock are described on page 187. 



B. From an elevation of 1,100 feet on the south slope of the 

 Korotini Range. . . . The description of the locality will be found 

 on page 160. 



This rock is hard and compact and looks like an altered basic 

 rock showing a few minute specks of pyrites. It is composed of 

 fine palagonitic debris, and small fragments of semi-vitreous basic 

 rocks and of crystals of pyroxene and felspar, none of the 

 fragments exceeding '2 mm. in diameter. Tests of minute 

 foraminifera, filled with matrix and of the " Globigerina " 

 type, occur very scantily. There is little or no carbonate of lime ; 

 but secondary silica, both colloid and crystalline, is present as an 

 alteration product. 



C. From the vicinity of Yaroi, 30 feet above the sea. . . . The 

 locality is described on page 189. 



This is a dark grey hard compact rock, containing probably 

 between 10 and 15 per cent, of carbonate of lime, and looking like 

 an altered limestone. In the section it displays minute tests of 

 foraminifera of the " Globigerina " type in a matrix composed of 

 fine disintegrated palagonitic material, impregnated with calcite 

 and containing also fragments of minerals (augite and felspar), 

 none of which exceed -2 mm. in diameter. There are also a few 

 similar-sized fragments of semi-vitreous basic rocks. Some fine 



