Here's why:

* High-Grade Metamorphism: The central Adirondacks experienced intense heat and pressure during the Grenville Orogeny, a mountain-building event about 1 billion years ago. These conditions led to high-grade metamorphism, which is the transformation of existing rocks into new ones with distinct mineralogical and textural characteristics.

* Gneiss Formation: Gneiss is a high-grade metamorphic rock characterized by its distinct banding. This banding is caused by the alignment of minerals under extreme pressure. The original rocks that were metamorphosed into gneiss in the central Adirondacks were sedimentary rocks like sandstone, shale, and limestone.

While gneiss is the dominant rock type, you might also find other metamorphic rocks like:

* Schist: A foliated metamorphic rock with a finer grain size than gneiss.

* Marble: A metamorphic rock formed from limestone, often found in smaller areas within the central Adirondacks.

* Quartzite: A metamorphic rock formed from sandstone, also found in smaller areas.

The Adirondack Mountains are a fascinating example of how tectonic forces can create diverse and beautiful landscapes.