You are describing the process of metamorphism, where rocks are transformed by heat, pressure, and chemically active fluids. The sequence of rocks you are referring to is known as a metamorphic facies series.

Here's a breakdown of the common metamorphic facies series starting with clay, which transforms into shale, and then progresses with increasing metamorphic grade:

* Shale: The initial sedimentary rock formed from compacted clay.

* Slate: A fine-grained metamorphic rock with a slaty cleavage, formed from shale under low-grade metamorphic conditions.

* Phyllite: A metamorphic rock with a slightly coarser grain size than slate, showing a silky sheen.

* Schist: A metamorphic rock with a distinct, planar foliation (layers) due to the alignment of platy minerals like mica.

* Gneiss: A high-grade metamorphic rock with a banded appearance due to the segregation of light and dark minerals.

* Migmatite: A mixture of igneous and metamorphic rock, often formed at very high metamorphic grades.

Note: The specific metamorphic facies that develop depends on the original rock composition, pressure, temperature, and the presence of fluids. There are many other metamorphic facies beyond these examples.

Example:

Imagine a clay layer buried deep within the Earth. As it gets buried deeper, the pressure and temperature increase. This can cause the clay to transform into shale. With further burial, the shale is metamorphosed into slate, then phyllite, and so on. Eventually, if the temperature and pressure are high enough, it could even turn into gneiss.