It should be smaller than the basal zone and generally it is. Sill then changes compoistion upward due to the differentiation process. Assimilation of Country Rock - Not thought to be important because the contacts between host and pluton are too sharp and inclusions are lacking. Mixing - Also not important as this would invalidate the systematic chemical variations. Rhythmic layering thought to be the result of repeated reinjection of new batches of magma, but cryptic layering invalidates this.
Graded bedding implies gravity settling.
Cryptic layering suggests fractionation is the dominant process. Local cross beds and slump structures indicate the magma chamber was not static, but rather undergoing convective motion. Despite its great age it remains largely intact and has been heavily studied. The complex consists of four lobes, which were originally thought to represent a single magma chamber. It is now generally agreed, however, that at least three separate magma chambers evolved concurrently.