Chocolate-coloured Foraminiferous Palagonite-marls
We have here hard, somewhat calcareous, clay-rocks which consist in great part (nine-tenths) of fine palagonite debris with some fragments of minerals and a little fine detritus of semi-vitreous basic rocks. Some hand-specimens would be taken for pure palagonite; but the fragmental nature appears at once in the slide. This is especially the case with a rock exposed in a stream-course near Rewa on the shores of Savu-savu Bay (see page [95]). The materials composing them are exceedingly fine, the largest fragments not usually exceeding ·2 mm. As a rule they contain a little carbonate of lime and sometimes as much as 10 per cent., whilst a few tests of minute foraminifera are to be noticed in the slide. These deposits are horizontally bedded, and underlie a pteropod-ooze rock at Nandua and a shelly impure limestone at Tembe-ni-ndio. They are not very frequent, and sometimes approach in characters the volcanic-mud rocks, which, however, are much more mixed in composition. I regard them in the main as sedimentary deposits derived from the disintegration of the palagonitised vitreous surface of a submarine basaltic flow. They pass downward at Nandua, as described on page [345], into a rock of pure palagonite; and they are only to be found in localities where basaltic plains or plateaux are covered over with submarine deposits.