Consanguinity in an assemblage of rocks, associated in time and place, is held to indicate that it has been derived by a more or less uniform geological process from a common initial magma. The most fundamental grouping of igneous rocks is thus based on distributional relations, which are in turn dependent on geological process and event.
The close association of igneous activity with earth movement is well known. The repetition of petrographic “provinces” or “kindreds” in time and place is clearly a manifestation of the cyclic character of tectonic events in geological history. The tectono-igneous cycle is elaborated and discussed with special reference to Scotland and Northwest Europe, and comparisons are made with eastern North America and Indonesia.