XXIII VOLCANIC MUD-ROCKS 321 



number of curious little pellets of palagonite, oblong or oval in 

 form, occur in the residue. Their size is ^ to '6 mm., and they 

 apparently represent the minute amygdules of palagonite that 

 occupy the vacuoles in an altered basic glass. 



FORAMINIFEROUS VOLCANIC MUD-ROCKS 



These deposits, which represent the " volcanic muds " forming 

 around the coasts of volcanic islands, are more or less consolidated 

 clay-rocks. They contain in varying numbers the tests of foram- 

 inifera with occasionally pteropod-shells. The former are usually 

 minute and of the " Globigerina " type ; but in some rocks larger 

 bottom-forms prevail. The original colour of these deposits is 

 bluish-grey, but as generally displayed they are pale-brown and 

 considerably affected by hydration and are known as " soapstones " 

 in the group. The ultimate effect of exposure is the production of 

 a whitish or yellowish soapy rock that has lost all the carbonate of 

 lime and all the organic remains and breaks down easily in the 

 fingers. Such a crumbling material, when examined with a high 

 power, cannot be distinguished from the products of the final 

 disintegration of palagonite as described on page 348. 



These deposits are frequently displayed in the lower regions up 

 to elevations of 300 feet above the sea, incrusting the basaltic plains 

 of Lekutu, Sarawanga, Ndreketi, and Lambasa, that occupy 

 such a large area of the north side of the island, and exhibiting 

 there in the prevailing horizontality of their beds the same indica- 

 tion that is presented by the vertical position of the columns of the 

 under-lying basalt, namely, the comparative absence of disturbance 

 during the emergence. At the foot of the mountains these 

 deposits are interstratified with and finally overlaid by coarse 

 palagonite-tuffs, also containing marine remains ; and these are in 

 their turn covered over by the agglomerates that often enter so 

 largely into the composition of the mountainous backbone of the 

 island. Such beds form apparently the lowest of a series which 

 begins with the foraminiferous clay and ends with the agglomerate. 

 But in some places, as has been noticed in the cases of the 

 pteropod-ooze rock of Nandua, and of the shelly and foraminiferous 

 limestone of Tembe-ni-ndio, beds composed largely of characteristic 

 palagonite lie beneath. 



In the elevated interior of the island these volcanic mud-rocks 

 ire usually concealed by the tuffs and agglomerates. Occasionally, 

 lowever, they are to be seen exposed by landslips high up the 



Y 



