The layer of rock that comprises 67% of the Earth's mass is the mantle.

Here's a breakdown:

* Crust: The outermost layer, making up only about 1% of Earth's mass.

* Mantle: The thickest layer, extending from the crust to the outer core. It accounts for about 67% of Earth's mass.

* Outer Core: Composed primarily of liquid iron and nickel, representing around 30% of the Earth's mass.

* Inner Core: A solid sphere of iron and nickel, making up the remaining 2% of Earth's mass.