Here's why:

* Schist is a metamorphic rock formed from the transformation of existing rocks (igneous, sedimentary, or even other metamorphic rocks) under intense heat and pressure.

* Basalt is a common, fine-grained igneous rock rich in minerals like feldspar, pyroxene, and olivine.

* Metamorphic Process: When basalt is subjected to high temperatures and pressure, its minerals recrystallize and align themselves in a parallel arrangement. This alignment gives schist its characteristic layered or foliated appearance.

While other igneous rocks can potentially be transformed into schist, basalt is a particularly common source rock due to its abundance and composition.