The idea that a batholith may make extensive progress upward by stoping its roof2 is now widely accepted. It is generally agreed that blocks of the roof become incorporated in the magma and that they are likely to undergo some corrosion or assimilation if they fall into the magma before it reaches the last stage of solidification. It is perhaps not so generally agreed whether the blocks are large or small, whether many sink to great depths, or whether the assimilation is so extensive that it may produce notable effects in the upper zones of the batholiths. later to be exposed to observation.
It has long been known that the Duluth gabbro lopolith contains, in its lower part,3 an abundance of large fragments, but there has been some confusion as to the nature of the material of these masses . . .