The extrusive igneous rock with a mafic composition and glassy texture is Tachylite.

Here's why:

* Mafic Composition: This means the rock is rich in magnesium (Mg) and iron (Fe), giving it a dark color.

* Glassy Texture: This indicates the rock cooled very quickly, preventing mineral crystals from forming.

Tachylite is often found as a thin, black glassy layer associated with volcanic eruptions. It is formed when very hot lava is ejected into the air and cools rapidly.