The transition from the earth’s crust to the mantle is called the ‘discontinuity of Mohorovicic’. The stones that are found below this layer consist of magnesium, like basalt, but are generally heavier and contain larger amounts of iron. The earth’s crust and the first 100 kilometres of the mantle are together referred to as the ‘lithosphere’. Generally, the variation in the stones gets less the deeper you get into the lithosphere. The layer below the lithosphere is called the ‘asthenosphere’. This layer is situated between a depth of 100 and 500 kilometres and contains even fewer hard stones. For this reason, stones in the asthenosphere can move around far easier than those in the lithosphere.