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Under these pressures, compaction is completed and the sediments
are transformed into solid rocks. The sands become sandstones, the
muds become shales, and the limey oozes become limestones.
Rocks formed from sediments, whatever marine or non-
marine, are naturally referred to as sedimentary rocks because they
are usually laid down in layers or beds or strata. They are also
called stratified rocks.

Igneous and metamorphic rocks

Some rocks were not formed from sediments but have been
melted and have solidified from a molten state. These are the
igneous rocks. Those that have reached or nearly reached the
surface while still molten are called lavas; they form volcanic
cones, or spread out in flows or sheets, or insert themselves as sills
between beds of other rocks, or squeeze and melt their way up
through other rocks to solidify as dikes. Rocks that have solidified
far beneath the surface are called plutonic. Granites the most
widespread igneous rock; it is usually a plutonic rock rather than a
lava. Basalt and rhyolite are common lavas.
When either igneous or sedimentary rocks are subjected to
enough heat and pressure their character and appearance are
changed. Either granitic rocks or masses of interbedded shales and
sandstones become gneisses and schists, sandstones become
quartzites, shales become slates, and limestone becomes marble.
All of these rocks thus formed by the metamorphosis of other rocks
are called metamorphic rocks.
Igneous and metamorphic rocks are important to the oil
geologist only because they form the basement complex beneath the
sedimentary rocks in which oil may be found, and because the
debris from their erosion has furnished a large part of the
sediments from which sedimentary rocks are formed. Otherwise,
igneous and metamoiphic rocks are anathema in oil geology;
except in rare and special instances, no oil is found in them.

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