Organic Deposits are seldom important in large lakes, but often decidedly so in small ones. As we have already seen, peat often forms to such an extent as to choke up the lake and convert it into a bog. Siliceous accumulations are made on an extensive scale by the minute plants, diatoms, which though of microscopic size, yet multiply with extraordinary rapidity; their tests of transparent flint gather on many lake bottoms in a fine deposit, as white as flour, and variously called Tripoli or polishing powder, or infusorial earth. Calcareous accumulations are formed by the shells of fresh-water molluscs, often in masses of considerable thickness. The lower layers of this shell marl have generally been so much disintegrated by the water as to be without any obvious organic structure. Such marls are frequently found under peat bogs and indicate that the latter were originally lakes, and in the marl often occur the bones of extinct animals.