DIABASE, in geology, a term which has at different times carried so many meanings that it has now lost all definite signifi cance. Originally it was applied on the Continent to many pre Tertiary basic igneous rocks, mostly of extrusive character. Later it was widely accepted as the group-name for the basic hypabyssal intrusives now called dolerites. The term is still used fairly often, especially in the literature of mining geology, and is useful as a field-name, if it is clearly understood as indicating only a some what decomposed basic igneous rock whose exact nature and origin has not been determined by precise methods. In this sense it is much the same as greenstone (q.v.).
The minerals of diabase, have been classed as the same as those of dolerite, viz., olivine, augite, and plagioclase felspar, with subordinate quantities of hornblende, biotite, iron oxides and apatite. To nearly every variety in composition and structure presented by the diabases a counterpart can be found among the Tertiary dolerites. In the older rocks, however, certain minerals are more common than in the newer. Hornblende, mostly of pale green colours and somewhat fibrous habit, is very frequent in diabase; it is in most cases secondary after pyroxene, and is then known as uralite; often it forms pseudomorphs which retain the shape of the original augite. Where diabases have been crushed or sheared, hornblende readily develops at the expense of pyroxene, sometimes replacing it completely. In the later stages of alteration, the amphibole becomes compact and well crys tallized ; the rocks consist of green hornblende and plagioclase felspar, and are then generally known as epidiorites, or amphibo lites. Chlorite is also abundant both in sheared and unsheared diabases, and with it calcite may make its appearance, or the lime set free from the augite may combine with the titanium of the iron oxide and with silica to form incrustations or borders of sphene around the original crystals of ilmenite. Epidote is another secondary lime-bearing mineral which results from the decompo sition of the soda lime felspars and the pyroxenes.
Diabases are exceedingly abundant among the older rocks of all parts of the globe. They form excellent road-mending stones, being tough, durable and resistant to wear, so long as they are not extremely decomposed.