iv MBUA SHELL-BED 59 



Wesleyan Mission Station and about two miles in a straight line 

 from the sea. This bed, which is about a foot in thickness, is ex- 

 posed for a distance of 70 or 80 yards. It slopes gradually sea- 

 ward as one descends the river, being raised two or two and a half feet 

 at its upper end above the river level at low tide, whilst at its lower 

 end it is at about the water-level. The river-bank is here 15 or 16 

 feet high, and is composed in its upper half of a fine gravel of 

 volcanic rocks mixed with earth, which below passes abruptly into 

 a friable non-calcareous black mud-rock (not bedded and looking 

 like consolidated swamp mud), in which the layer of shells is con- 

 tained. These shells are, therefore, covered by deposits, 13 or 14 

 feet in thickness, of which the upper eight feet are formed of gravel 

 and earth, and the rest of mud-rock. They are evidently gathered 

 together on the slope of an old mud-fiat. 



The shells are all large marine bivalves, belonging to the genera 

 Ostraea, Meleagrina, Cardium, Area, &c, no freshwater shells 

 occurring. They are often much decayed and have lost the liga- 

 ments. The valves are generally separate ; but in some cases they 

 are still in apposition, the cavity being then filled with the same 

 black mud in which the shells are embedded. They lie about in 

 all positions, some vertical, some horizontal, and are often piled on 

 each other. In some cases large borers have perforated one or 

 both of the valves ; and here and there valves may be noticed with 

 smaller oyster-shells attached to the inner surface. No vegetable 

 remains were discovered with the exception of a single " stone " of the 

 fruit of the Sea tree, 1 which is common in'these islands, its empty 

 almost indestructible stones occurring frequently in the drift stranded 

 at the mouths of rivers. 



At first sight one would look to human agency for the explana- 

 tion of this shell-bed ; but many of its features are inconsistent 

 with such a view. If the shells had been originally collected by 

 the aborigines for food, the absence of those of marine univalves of 

 the genera Turbo, Strombus, Cypraea, &c, such as are much 

 appreciated as food by natives, is inexplicable. The extent of the 

 bed and its uniform thickness are characters that give no support 

 to such an explanation. It represents, as I apprehend, an ancient 

 shell-bank formed on a muddy bottom in comparatively shallow 

 water near the mouth of a river. Since that time the Mbua River 

 has cut through its old deposits, and the margin of its delta is now 

 two miles to seaward, the intervening new land being formed 

 of extensive mangrove-swamps in its lower part, whilst nearer the 



1 Species not identified. 



