Nine laccoliths to the east and northeast of the Highwood Mountains are described. With one exception they have been intruded into the upper Cretaceous Eagle sandstone. Varying degrees of erosion in the different laccoliths permit some to be studied in more detail than others. Discussion centers around Shonkin Sag laccolith since it is of most general interest and is most completely exposed.
All the rocks of the different laccoliths show the same mineral assemblage; the chief difference between them is a variation in the percentages of the minerals present. The only major exception is the hornblende-sodalite syenite from the top of Square Butte. In four laccoliths a horizontal layer of syenite is present with a thick zone of shonkinite below and a thinner zone of shonkinite above. In three other laccoliths, syenite is found above shonkinite, but the upper contacts are eroded. In several laccoliths an increase in density upward through the lower shonkinite was observed. In addition to this variation, Shonkin Sag laccolith shows a decrease in density downward through the upper shonkinite.
Field and laboratory observations made on all the laccoliths are explained on a theory of differentiation in place. This theory is based on the settling of heavy crystals accompanied by minor rising of leucite in a cooling body.