An intrusive rock, similar to dolerite, but with an excess of quartz. Dolerite is similar in composition to basalt, which is eruptive (from volcanoes), and gabbro, which is plutonic. The differing crystal sizes are due to the different rate of cooling, basalt cools quickly and has a very fine structure, while gabbro cools very slowly, at great depth, and large crystals develop. Dolerite is intermediate.