Here's why:

* Intrusive Igneous: Batholiths form when magma rises from deep within the Earth's crust and cools slowly underground. This slow cooling process allows large crystals to form, resulting in igneous rocks like granite, diorite, or gabbro.

* Size Matters: The defining characteristic of a batholith is its massive size. To be classified as a batholith, the igneous intrusion must be at least 100 square kilometers (40 square miles) in area.

* Variety of Composition: The specific type of rock within a batholith depends on the composition of the original magma. This means a batholith can be made up of granite, diorite, gabbro, or any other type of intrusive igneous rock.

So, while a batholith isn't a specific type of rock, it's a significant geological structure made up of various intrusive igneous rocks.