20 ANNIVERSARY ADDRESS. 



tion. This manganese is peculiarly set free, like the iron, by the 

 decomposition of the organic bodies and tests. In some Alga' it 

 exists to the amount of 1 per cent. 



A shark's tooth is sometimes replaced by the manganese and a 

 piece of pumice or a pebble is coated with a fine black mammil- 

 lated layer.* 



Professor TVyville Thomson remarks that, though the red clay 

 is unfavourable to life, it is possible at all depths, and diminishes 

 below 1,500 fathoms. Moreover, the free carbonic acid found 

 in all sea water is rather in excess in the greater depths ; and it 

 is possible that the sea water may attack the sinking shells, and 

 begin to decompose them in their slow descent to the bottom 

 through a passage of a half-mile or so of water of increasing 

 density. Nevertheless HolotJiurids and Bryozoa were sometimes 

 drawn up from great depths, and about 150 miles from Sombrero 

 very well-marked red mud which did not effervesce with hydro- 

 chloric acid held imbedded " tubes of a tube-building Annelid, 

 several of which were from three to nine inches long, containing 

 the worm still living." 



The conclusion drawn by the Professor is, that as the chalk, 

 so the red clay may be under certain circumstances an organic 

 product, and that " an area of the surface of the globe, which we 

 have shown to be of vast extent, is being covered by such a deposit 

 at the present day." 



He also suggests that " the fine, smooth homogeneous clays and 

 schists, poor in fossils, but showing worm tubes and tracks, and 

 such things as Oldhamia, siliceous Sponges, and thin-shelled 

 peculiar Shrimps (belonging to the oldest known geological 

 sediments) may be organic formations like the red clay of the 

 Atlantic and Southern Sea — accumulations of the insoluble ashes 

 of shelled creatures." 



* This deposit of manganese, of which. I saw specimens on board the 

 " Challenger," is very common on pebbles and quartz in some of the alluvial 

 auriferous deposits on the Cudgegong, and is also seen on smooth planes of 

 granite, porphyry, sandstone, &c, &c, in the form of Dendrites. 



