The parent rock effect in the genesis of soil

Weathering trends emanating from the compositional field of the common igneous rocks, can be shown graphically to converge on compositions relatively rich in SiO₂, Al₂O₃ and Fe₂O₃. This convergence means that in chemical terms, the effect of the composition of igneous parent rock on the composition of the resulting soil, is an inverse function of time. Given enough time the chemical effect of parent rock would be nullified. The consequences of this to “state factor equations” are indicated.