Basalt is a volcanic rock probably formed by the fluid magma escaping through some line of fracture in the earth's crust, overflowing at the surface, and then cooling slowly. Being a volcanic rock, it shows a porphyritic structure - that is, crystals are embedded in a fine-grained ground mass. This is owing to the molten magma solidifying slowly |near the surface and giving time for some of the minerals to crystallise out. Basalt is essentially a plagioclase feldspar rock with augite or hypersthene, and mayor may not contain olivine, thus giving basalts and olivine basalts. In the older basalts the olivine is often decomposed into serpentine, and gives an amygdaloidal structure to the rock. The minerals found in basalt are plagioclase feldspar, augite, hornblende, and sometimes small crystals of quartz and cdivine. Under the microscope are seen many lath-shaped crystals of plagioclase feldspar, being easily detected with crossed Nichol prisms by its polysynthetic twining, which results in the development of a series of parallel bands of colour crossing the gram Augite is pale brown in colour, and when revolved on the stage of the microscope, using only the lower Nichol prism, the colour does not change.

Olivine is very pale greeu in colour, and generally traversed by cracks which are more or less decomposed into serpentine. Having a higher index of refraction than the augite, the olivine appears more prominently.