Diatomaceous earth quarry. Exposed in this quarry near St Bauzile in mid-eastern France is a soft, white sedimentary deposit called diatomite. The diatomite is formed by the deposition, in a lake or inland sea, of the tiny, siliceous tests of myriad planktonic microorganisms, single-celled protozoans known as diatoms or radiolarians. The diatomite is overlain by dark brownish basalt, a massive volcanic rock, here solidified in a columnar mode. The diatomite, a source of very pure silica, is mined from this quarry.