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1 March 2011
News from the Billy Goat Trail (3): grab bag
Okay, a final post (for now, anyhow) sharing some images from last Friday’s field trip to the Billy Goat Trail, in Potomac, Maryland. Yesterday we looked at lamprophyre dikes, but there are other dikes on the Billy Goat Trail, too. Like this granite pegmatite: What’s interesting to me about this is that the joint set seems to have contradictory cross-cutting relations with the dike. Did the joints come first? Or …