Oh, Schist!

The eastern edge of Twin Island, facing Long Island Sound just north of Orchard Beach in the Bronx, is an outcropping of the Hartland Formation schist.And does it ever outcrop!Quoting the geological argot of the USGS “The rock consists of granitic and garnetiferous amphibolite gneiss with numerous quartz veins and migmatite dikes. Migmatite is an type igneous rock that forms when metamorphic rocks begin to melt under high temperature. Felsic minerals melt and are injected into the surrounding rock along joints, faults, and other zones of weakness in the rock. As the igneous material gradually cools, bands of feldspar and quartz crystals form along the edges of the intrusion. The center of the migmatite veins typically consist of larger crystals of feldspar and quartz. The migmatite stands out in outcrops as light-colored bands in contrast to the darker amphibolite gneiss host rock. In some cases, the dikes cut across older dikes and quartz-filled veins; many are folded or display offset by faulting.” It’s an open-air classroom, so here’s Hunter College on the spot: “The site contains the following rock types: gneiss, amphibolite, migmatite, pegmatite, quartz veins, marble, ptygmatic folds, pearl gneiss, and boudines. The following minerals can be found here: K-feldspar, quartz, biotite, muscovite, hornblende, garnet, pyrite, limonite, kaolinite, calcite, tourmaline, plagioclase, and chlorite.” .I’ll say that again!

3 Responses to “Oh, Schist!”


  1. 1 Paul Lamb January 25, 2019 at 5:46 am

    Wow! This brings back memories of the rock hound in me. Nice post.

  2. 2 Sherry Felix January 25, 2019 at 8:24 am

    I love geology and taught geology lab at Hunter while an adjunct. A few of the rocks you have labeled schist I would call gneissic schist because many are well on the way to becoming non-foliated rocks, composed of minerals that have recrystallized into solid, interlocking networks. It’s a mixed bag of rocks up there. I love that place.


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