Rocks with grains arranged in parallel layers are called foliated rocks.

Here's why:

* Foliation: This refers to the arrangement of mineral grains in a planar (flat) or layered structure.

* Parallel layers: The mineral grains align themselves in parallel layers due to pressure and heat during the rock's formation.

Some examples of foliated rocks include:

* Slate: Fine-grained, with a flat, parallel layering.

* Schist: Medium to coarse-grained, with a more pronounced layering.

* Gneiss: Coarse-grained, with distinct bands of light and dark minerals.

Let me know if you'd like to learn more about any specific foliated rock!