These rocks have no felspars, and in most of them the quantity of silica is below 45 %, while that of magnesia is from 35 to 48 %; they are composed almost entirely of ferro-magnesian minerals.

Limburgite is made up of crystals of augite, olivine, and magnetite, embedded in a glassy ground mass.

Augitite is a similar rock, but without olivine.

Pyroxenite is a holocrystalline, plutonic rock composed of one or more varieties of pyroxene.

Hornblendite is a similar rock made up of hornblende.

The Peridotites are likewise plutonic rocks which are principally composed of olivine, with iron ore and some of the pyroxenes or hornblende.

The Serpentines are products of alteration, and many of them have been formed from peridotites, though some have been derived from augitic rocks, such as gabbro, and others from hornblendic rocks. In rarer instances they have arisen from the alteration of acid rocks.