La Posta Soil Profile #2 developed on a small area underlain by the small-biotite facies of the Cretaceous La Posta pluton within an arid climatic belt. Within La Posta Soil Profile #2, R, C, and A weathering horizons are apparent. Five samples for chemical analysis were collected from each horizon. On the A-CN-K ternary diagram, resulting data form a crude linear trend sub-parallel to the A-CN join. Moreover, data from the Ahorizon consistently plot above data derived from R and C, and closer to the A apex. On the A-CNK-FM ternary diagram data form a crude linear trend directed toward the A-FM join, and away from the CNK apex. Again data from the A-horizon plot away from data from Rand C, and closer to the A-FM join. Bivariate plots of Sr, Ca, and Na show that these elements are depleted in the A-horizon relative to horizons Rand C. All three elements are common to plagioclase. Data discussed above indicate that plagioclase is being degraded and removed from the A-horizon during its development.
