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materials for sediments and they may also be involved in 
tlie disintegration and decomposition, transportation, and 
deposition of other material. Fossil remains of animals 
and plants are often incorporated in sedimentary rocks. 
Some Common Sedimentary Ttoclcs. 
The materials involved and the variety of processes 
operating in the formation of sedimentary rocks result 
in a great diversity of end products. However, some 
occur more commonly than others. 
Mudstone and shale are rather similar kinds of 
sedimentary rock. They are formed from the accumula- 
tion of the finest transported products consisting essen- 
tially of clay minerals, which are produced as decom- 
position products from most rocks. Shale is subjected to 
a greater degree of compaction than mudstone; it is 
harder and tends to split more readily along the bedding 
planes. 
Sandstones consist mainly of the mineral Quartz. 
Quartz, which is one of the most common minerals in the 
earth's crust, is not easily decomposed and is not readily 
reduced below the sand grain size by natural abrasion. 
The cementing material in sandstones may be clay, silica, 
calcium carbonate or iron oxide and the last of these is 
usually responsible for their yellow, red, or brown colour. 
Conglomerates are composed of large rounded rock 
fragments cemented with finer material. They are pro- 
duced under vigorous conditions of erosion and trans- 
portation, but because of the particle size, the distance 
of transport is not great. 
Tillite is the name given to the rock produced hv 
the consolidation of boulder clay, which is produced by 
glacial erosion. As the name implies boulder clay is an 
heterogeneous aggregate and this may he readily under- 
stood by consideration of the mode of formation. 
Limestone is a sedimentary rock consisting essen- 
tially of the mineral Oaleite, chemically calcium car- 
bonate. Much of this material is deposited chemically. 
However, some limestones are composed partly of the 
calcareous skeletons of organisms such as corals. In some- 
cases whole reefs may be buried. 
Coal is formed from vegetable matter which is de- 
posited in lakes and passes through a peat stage. The 
formation of black coal from peat involves chemical 
changes as well as compaction, after burial by other sedi- 
ments. Under the prolonged influence of heat and 
pressure, the carbohydrates of the buried vegetable 
