Photo by Warren Lane
Above: In this photo, taken along the Highline Trail about 5 miles northwest of Logan Pass, I am standing at the upper boundary between the sill and the marble. Here, contact with the limestone caused the magma to cool fairly quickly, resulting in smaller crystals (grains) in the diorite. The middle part of the sill (not seen here) has larger crystals that formed as the magma cooled more slowly.
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.