Below: In this photo taken along the Highline Trail about 2 miles northwest of Logan Pass I am standing in front of the upper portion of the sill - The bottom portion is not exposed.
Right: A fellow hiker points out the contact (boundary) between the sill and the marble. The light-colored rock above the contact is limestone (or dolomite) of the Helena formation that were baked by heat from the magma, driving the organic material away. Contact with the limestone caused the magma to cool fairly quickly, resulting in smaller crystals (grains) in the dark-colored diorite.
Most geologists call the dark rock that makes up the sill "diorite" but in some places it appears to be more like "diabase". Diorite is a medium-colored, course-grained igneous rock whose mineral content is between basalt and granite, whereas diabase is a course-grained igneous rock whose mineral content is more similar to basalt.