Rivers and ocean waves constantly erode the land, and carry large quantities of sand and mud far out to sea to be deposited on the bottom in layers. Myriads of minute organisms, some living a benthic existence on the sea bottom, others floating in the upper layers of the sea as plankton, leave behind hard skeletons and shells which accumulate on the sea bed and become entombed in the sediment along with minute skeletal parts of larger organisms. Over a long period of time the layers of sediment become compacted and hardened to form rock strata, and may be exposed as dry land in a later geological period by withdrawal of the sea or uplift.

Rock strata, like the leaves of a book, are the pages of geological history which can be read by the micropaleontologist who studies assemblages of microfossils preserved in successive beds. As an archaeologist can assign artefacts to certain periods of human culture or a historian identify the characteristics of a period of history, so can a micropaleontologist tell in which geological age an assemblage of microfossils from a particular stratum was buried in the sea bed. Microfossils therefore help the field geologist to work out correctly the sequence of rocks and their geological structure in areas where the earth's crust is strongly contorted and dislocated or obscured by vegetation or young deposits.

New Zealand sedimentary rocks of upper Cretaceous and Tertiary age are rich in microfossils and there are numerous publications dealing with them.

Planktonic microfossils (freely floating in life) are widely distributed throughout the oceans by currents and similar assemblages of species are found in rocks of the same age in different parts of the world. Planktonic forms are therefore particularly useful for correlating rocks of the same age throughout the world and they help geologists to piece together more accurately the story of geological events in the earth's history.

Microfossils are used by oil exploration geologists in their surveys of possible oil fields. Their abundance and small size makes them especially easy to collect and store. A pea-sized piece of soft limestone or limey mudstone may contain many thousands of microfossils. Most soft limestones, mudstones or sandstones in New Zealand will yield Foraminifera, Ostracoda or Bryozoa if they are first well soaked and then washed over a fine sieve with holes of a millimetre or less in diameter.

by Norcott de Bisson Hornibrook, M.SC., F.R.S.N.Z., Senior Principal Scientific Officer, New Zealand Geological Survey, Lower Hutt.

• The Tertiary Cheilostomatous Polyzoa of New Zealand, Brown, D. A. (1952)
• New Zealand Geological Survey Paleontological Bulletin 18, Tertiary and Recent Marine Ostracoda of New Zealand, Hornibrook, N. de B. (1952)
• New Zealand Geological Survey Paleontological Bulletin 34, Tertiary Foraminifera from Oamaru District (N.Z.), Hornibrook, N. de B. (1961)
• Introduction to Microfossils, Jones Daniel, J. (1956).