Locke was an empiricist in that he thought that

I believe the answer is all knowledge originates in sense experience

Empiricist refers to the type of people who based all his knowledge based on their personal experiences rather than following a certain manual or guideline.
This type of thinking is part of three types of human knowledge mechanism, along with rationalism and skepticism.

The one that most represents the earths mantle would be the skin since the mantle is the upper part of the earths depths which is just like the skin of an apple.