A historic look at the geology of the Isles of Shoals

Continuing from our last post using Frank Preston Stearns’s Sketches from Concord and Appledore  of1895, now a look at the geology of the islands:

Their geological structure is more interesting. It is generally supposed that the soil of New England rests on a foundation of primeval granite but it is not exactly that. There is very little true granite in New England, what is taken for it commonly being syenite, a rock indeed that differs from granite only in the substitution of horn-blend for mica.

Syenite

Syenite

Above Stearns is describing syenite which modern geologists describe as a coarse-grained intrusive igneous rock of the same general composition as granite but with the quartz either absent or present in relatively small amounts (<5%).

The so-called Quincy granite is a finer sort of syenite, which in the White Mountains are composed of syenite kept with granite. The Isles of Shoals are also mostly syenite, but there are large boulders of coarse granite lying about, and in some places the syenite changes suddenly to granite as if the two had been welded together. Then there are dykes of dark brown trap or ancient lava, from four to ten feet wide running across the island from south-west to north-east, and others again at right-angles to these.

If you recall, or look back to the last post, you’ll notice that Stearns comments that the Islands were never mountain tops. Here he describes how they may actually have formed due to volcanic action millions of years ago.

This would seem to indicate that the elevation of the surrounding plateau was due to volcanic action. The structure of White Island is very different from the others, a large portion of the rock being studded with innumerable garnets, while veins of some greyish white minerals run through it in which there are still smaller garnets.

Sterans, Frank Preston (1895) Sketches from Concord and Appledore. G.P. Putnam and Sons: New York.

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