In this image we see the layering of crystals that accumulate and precipitate out of the melt in a magma chamber. There are light colored trondhjemites and mafic peridotite but most of the cumulate in the chamber is a coarse gabbro. So in this image the layering is not by tectonics or metamorphism but rather my crystals setting out of the melt through time. The mafic minerals precipitate first, then sequentially other minerals rain out of the melt and accumulate on the floor of the magma chamber. Banding or repeated crystal precipitation events suggests that the chemistry of the melt is changing in time as new magma enters the chamber or as the melt slowly cools. Though there can be variations due to the influx of new magma, the general progressive trend toward cumulates that are more feldspar rich provides geologists with good right-way-up indicators, an important tool for geologists mapping in field areas with extensive deformation. Image courtesy of Professor Burchfiel.