Figure 23. Left edge of a diabase dike, in sharp contact with coarse-grained granite. The diabase fills a vertical fracture which opened in the granite. There is a similar vertical fracture in the granite just to the left which might have opened, but didn't. The diabase is a sheet, which continues into the rock, away from the road. The very fine (microscopic) grain size of the diabase, and the brittle fracturing of the granite both indicate that the granite was cold at the time the diabase intruded. Similar diabase dikes in southern Maine are dated at about 200 million years old, which is about a hundred million years younger than the granite.
Last updated on August 18, 2010