The contact effects resulting from the intrusion of igneous material into various wall rocks are linear compositional changes only in special instances. Large variations in chemical composition from point to point may be present in both intrusive and wall rocks.
A study of these variations by the analysis of closely spaced samples with quantitative spectrographic methods indicates that all of the major rock constituents studied (Al, Fe, Mg, Mn, Ca, Na, and K) are mobile when moderate temperatures and concentration gradients are present. Migration of material is believed to take place late in the cooling history by diffusion along mineral boundaries. The compositional variations resulting from this transfusive migration of hydrated ions are often sufficiently large to mask any compositional changes due to earlier reactions.
The chemical variations found can be related to the original compositions of the wall rock and intrusive material, and to the cooling history of the system. No variations in chemical composition are found across contacts between chemically identical rocks. Generally smooth and continuous variations occur across contacts between rocks of different original composition which have been held at high temperatures. Highly irregular variations are found when the intrusive and wall rock were compositionally different and steep thermal gradients existed. These latter variations may show geochemical culminations and depressions are wave-like in form, and die out in both directions from the contact zone.