http://b4hntuy3fimfh2227vf4f74emnya7p35i5brtqujs6leqvtclfwvjbqd.onion/doc/physics/diamonds-crookes/The%20Project%20Gutenberg%20eBook%20of%20Diamonds,%20by%20William%20Crookes..html
The surface of the country round Kimberley
is covered with a ferruginous red,
adhesive, sandy soil, which makes horse
traffic very heavy. Below the red soil is a
basalt, much decomposed and highly ferruginous,
from 20 to 90 feet thick, and
lower still from 200 to 250 feet of black
slaty shale containing carbon and iron
pyrites.