Excerpt from Teachers' School of Science of the Boston Society of Natural History, Lowell Free Courses, 1889-90: Physical History of the Boston Basin

The remaining islands Of this group are almost or quite driftless, and form part of a great synclinal fold of slate, with intrusive beds of diabase. After the quiet deposition on the sea-floor of a great thickness Of clayey sediment or mud, which, by hardening, formed slate, the beds were compressed horizontally and bent so as to form a concave arch or syncline. And during the folding of the slate cracks were formed through which melted rock (diabase) rushed up from below and then spread out between the layers of slate, forming more or less regular sheets of diabase parallel with the slate above and below. Calf Island lies on the north side of the syncline and the Brewsters on the south side. Calf Island is crossed by two great intrusive beds, with a bed of slate about thirty feet thick between them. The lower or most northerly of these two beds of diabase must be several hundred feet thick and forms the whole of Little Calf and Green Islands, the Roaring Bulls and the Graves. Near the slate, especially, the diabase possesses a very coarse and perfect flow-structure; and it also exhibits on a large scale the concentric structure characteristic Of many massive eruptive rocks.

The best exposures showing the alternating beds of slate and diabase are on the south side of Middle Brewster Island. And since the igneous sheets of the outer islands are not only clearly and typically intrusive, but also admirably exposed for study, they merit the especial attention of all students of our local geology, as affording a valuable standard for comparison in determining whether the eruptive masses in other parts of the Boston Basin are intrusive or contemporaneous.

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