Anorthosite and granite or syenite are associated with masses of gabbro at so many places that many petrologists believe that they have a common origin, or that they are differentiates from a common magma. The great sill at Pigeon Point, Minnesota, shows some features that indicate rather clearly the genetic relations of such rocks, and that may throw light on the general problem of rock differentiation. This sill seems to have been large enough to permit thorough differentiation, but not so large that the intrusion of the later phases of the magma obscured the relations of the phases already solidified. The outcrops are numerous, and there may be no other single geologic unit in which the relations of these common rock types are so clearly and suggestively exposed.
The field-work was done for the Minnesota Geological Survey. The writer has had the advantage of frank discussion with . . .