Here's why:

* Igneous rocks are formed from the cooling and solidification of molten rock (magma or lava). They are often the oldest rocks found in a geological sequence.

* Sedimentary rocks are formed from the accumulation and cementation of sediments, which are typically derived from the weathering and erosion of older rocks (including igneous rocks). They form on top of existing rock layers.

* Metamorphic rocks are formed when existing rocks are transformed by heat, pressure, or chemical reactions. They can form from both igneous and sedimentary rocks, and their position in a sequence depends on the original rock type and the geological processes involved.

Therefore, since you are typically found on the bottom of rock layers, you are likely an igneous rock that formed first and was then covered by other rock types over time.