Here's why:

* Igneous rocks are classified based on their mineral composition, which is directly related to the chemical composition of the magma or lava from which they formed.

* Mafic rocks are rich in magnesium and iron, and they are typically dark-colored. Examples include basalt and gabbro.

* Felsic rocks are rich in silica and have a higher proportion of lighter elements like sodium and potassium. They are typically light-colored. Examples include granite and rhyolite.

* Intermediate rocks fall in between these two extremes. Andesite is a common example, formed from volcanic eruptions, and it has a composition that falls between basalt and rhyolite.

So, andesite is the rock with an intermediate composition.