Johannsen's work on petrography is both a descriptive petrography of the quartz-bearing igneous rocks and an illustration ... ratio of the light-coloured and dark-coloured minerals present ...

In this system rocks are first divided into classes according to the relative amounts of light- and dark-coloured minerals ... A Descriptive Petrography of the Igneous Rocks By Prof.

The dark colour of Skull Hill resembles meteorites ... Instead, it may be an igneous rock formed from magma or lava, containing iron and magnesium-rich minerals such as olivine or pyroxene.

While Skull Hill’s dark hue resembles meteorites previously ... Instead, scientists believe it may be an igneous rock formed from cooled magma or lava, rich in minerals like olivine and pyroxene ...