In caves, the basis of the food chain is organic matter from the surface.
It is markedly deficient in potassium and to some extent in phosphate and organic matter.
The blue colouring substance is ferrous sulphide, the upper reddish layer contains more ferric oxide, which the predominance of decomposing organic matter in the substance of the mud reduces to ferrous oxide and subsequently by further action to sulphide.
Red mud may be classed as a variety of blue mud, from which it differs on account of the larger proportion of ochreous substance and the absence of sufficient organic matter to reduce the whole of the ferric oxide.
A small proportion of organic matter including the fat globules of the plankton is mixed with the calcium carbonate, the amount according to Giimbel's analysis being about 1 part in 1000.