Which layer of earth acts as a very effective “insulator,” because it’s made of rock?

Earlier in the passage this question was provided for, you should be able to find the fallowing text: "The mantle, being made of rock, acts as a very effective insulator."

Mantle

common management practices in dairy and poultry farming are: (i) proper shelter facilities and their regular cleaning. (ii) some basic hygienic conditions such as clean water, nutritious food, etc. (iii) animals are kept in spacious, airy, and ventilated place.