Varieties of basaltic lava and tuff containing little or no olivine - the rock type known as labradorite.
Diabases pierce to Devonian rocks, and olivine rocks appear as dykes amidst the Triassic deposits.
In addition to the reddish or brownish argillaceous matrix it contains fresh or decomposed crystals of volcanic minerals, such as felspar, augite, hornblende, olivine and pumiceous or palagonitic rocks.
They are soft and lustrous, with a peculiarly smooth feel, and though often confounded with mica-schists may be distinguished by their richness in magnesia; many of them contain tremolite or actinolite; others have residual grains of olivine or augite; and here also every gradation can be found between the unmodified igneous types and the perfectly metamorphic schists.
Secondary product of olivine and certain pyroxenes.
Biotite and olivine are not really frequent in these rocks, and usually have been affected by resorption.