The sheets of igneous rock that are parallel to the layers they intrude are called sills.

Here's why:

* Sills are tabular (flat and sheet-like) intrusions of igneous rock that are concordant, meaning they are parallel to the layers of the existing rock they cut through. They form when magma squeezes between existing layers, often taking advantage of planes of weakness like sedimentary bedding.

* Dikes, on the other hand, are discordant intrusions, meaning they cut across the existing rock layers at an angle.

Let me know if you'd like more information on igneous intrusions!