I wonder if anyone can help me with this rock formation on the Back Shore. What is the black in the granite. It looks like it was poured in.
Jackie Bennett Is Curious About This Rock Formation On The Back Shore
My View of Life on the Dock
Good Morning Gloucester 
That is a basalt dike. The basic idea is that this magma like fine-grained rock was injected into and expanded a crack in pre existing granite.
But I geologist could tell you more!
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Thank you so much. I have noticed it along the shore before but I was fascinated by the way it looked like it was flowing down the crack in the granite. Jackie
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Yes, it’s a dike of, probably, diabase (a basalt relative). If you can find a copy of Martin Ross’s excellent “Cape Ann: Its Physical and Environmental Geology” he explains the many dikes that cut through Cape Ann granite.
If this is the one I think it is, you can also see it over on Andrew’s Point. Yes, it’s the same structure across the water.
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Basalt dike?
Fo example https://www.geologypage.com/2019/03/intrusive-basalt-dike-acadia-national-park.html
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