The layer of the Earth composed mainly of solid iron is the inner core.

Here's why:

* Earth's Structure: The Earth is made up of several layers: the crust, mantle, outer core, and inner core.

* Inner Core: The inner core is the Earth's innermost layer. It's incredibly hot and dense, with pressures so immense that the iron, along with some nickel, remains solid despite the extreme temperatures.

* Outer Core: The outer core, surrounding the inner core, is also primarily composed of iron and nickel, but it's liquid.

Let me know if you have any other questions about the Earth's layers!