Igneous rocks, unless cooled with singular rapidity, thus contain crystals of various kinds. In lavas, these may form the globular aggregates known as _spherulites_[62], or may accumulate as a compact ground of minute grains and needles, not quite resolvable with the microscope. In many rocks of slightly coarser grain, a compact _lithoidal_ or stony texture is set up, which the microscope resolves into an aggregate of crystalline rods or granules. Such compact rocks are often styled _felsitic_. In other types, as in ordinary granite, the constituent minerals are easily distinguished with the naked eye. The order in which these constituents have developed is sometimes clear from the inclusion of one mineral in another. When two substances are dissolved in one another, there is a certain proportion between them, varying with the substances, which allows them to crystallise at the same time, instead of in succession. This _eutectic proportion_, when attained by two mineral substances in a magma, brings about a complete interlocking of their crystals, as is seen in the quartz and alkali-felspar of the rock known as "graphic granite." The order of crystallisation of minerals from an ordinary non-eutectic magma is profoundly affected by the proportions in which their constituents are present in the mass. The minerals, when they have separated out, are found to be mostly silicates. A few oxides, such as rutile, magnetite, and ilmenite, may occur, the two latter being especially common where iron is an important constituent of the rock. But almost all igneous rocks consist largely of one or more species of felspar, silica being here combined with alumina, potash, soda, and lime. Free silica may remain, and separates as quartz, or rarely as tridymite. Pale mica occurs in many rocks of deep-seated origin. In contrast with these light-coloured minerals, iron, magnesium, and part of the calcium, appear in another series of silicates, usually dark in colour, and this series may be broadly styled "ferromagnesian." The pyroxenes, of which augite is the type, the amphiboles, of which hornblende is the type, dark mica (mostly biotite), and olivine, are the ordinary ferromagnesian minerals. Broadly, then, igneous rocks divide themselves by texture into (i) those which are completely crystalline, and in which the minerals are distinctly visible; (ii) those which are completely crystalline, but in which the crystals are so small as to give rise to a compact lithoidal ground-mass; and (iii) those in which some glass is present. The third group may appear lithoidal, or in other cases actually glassy, to the unaided eye.