Here's why:

* Mineral Composition: Shale is composed of very fine-grained clay minerals, which are generally denser than the minerals found in slate. Slate is primarily composed of mica, quartz, and feldspar, which are less dense.

* Compaction and Cementation: Shale forms from the compaction and cementation of mud and clay. This process squeezes out water and increases the density of the rock. Slate forms from the metamorphism of shale, which involves heat and pressure. While this process does increase density somewhat, it's not as significant as the initial compaction and cementation of shale.

* Porosity: Shale often has a higher porosity than slate. Porosity refers to the amount of empty space within a rock. The higher porosity of shale means that it contains more air pockets, which makes it less dense. Slate, with its more tightly packed structure, has lower porosity.

So, while both slate and shale are relatively dense rocks, shale is typically denser due to its composition and formation process.