Pieces of one rock unit contained within another are called

The correct answer is - intrusive rocks.

The intrusion is a process in which the rising magma in the Earth's crust, manages to break through pre-existing layers of rock, and after that cool off, solidify, and create new intrusive igneous rocks. By doing so, the magma is breaking up the layers of rocks, thus the intrusive igneous rocks that are forming from it, come to be inside totally different rock units. The intrusions are always happening inside the crust, and the rocks formed from them are igneous rocks that have large crystals because of the slow cooling off of the magma.

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