Here's why:

* Density: Shale is composed of minerals like clay, quartz, and feldspar. These minerals have a higher density than water.

* Porosity: While shale can have some pores, they are usually filled with water or other fluids. This doesn't significantly reduce the overall density enough to make it float.

So, even though shale might look like it could float, it's actually denser than water, causing it to sink.