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East Rock, New Haven, New Haven Co., Connecticut, USA

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Latitude & Longitude (WGS84): 41° 19' 35'' North , 72° 54' 14'' West
Latitude & Longitude (decimal): 41.3263888889, -72.9038888889

East Rock is a diabase sill and dike complex that intrudes the Triassic New Haven Arkose, forming a 2-mile-long, roughly N-S trending irregular ridge straddling the New Haven-Hamden border. Dana (1877) reports on the discovery of andradite garnets in a quarry just south of the summit:

At the East Rock locality the trap is very distinctly columnar, and the garnets occur on the vertical surfaces of the columns where they have been exposed in the process of quarrying. The spot at which they are found is in the body of the dike, far away from the line of contact with the sandstone; they are scattered here quite freely over a considerable surface; a careful search for them, however, along the extended front of the Rock, failed to reveal any other locality.

"The associated minerals are: Magnetite, apatite, pyroxene now altered to chlorite, calcite, and also in traces chalcopyrite and sphalerite.

"The garnets themselves are uniformly crystallized in rhombic dodecahedrons, with the edges truncated by the planes 2-2. Isolated crystals, however, are rare; more commonly they are grouped together in nearly parallel positions in very pretty little rosettes. These groups are scattered over the exposed surface of the rock, and are quite characteristic of the locality. Still, again, the crystals are crowded together, without any regularity of position, into crusts of some little thickness.

"The color of the garnets is generally dark-brown to jet-black, though occasionally they are yellowish-brown. Their luster is very brilliant, and in general they are entirely free from alteration. An analysis of the garnets proved, what indeed was suggested by their appearance, that they belonged to the variety melanite.

Mineral List

5 valid minerals.

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Dana, Edward S. (1877): On the occurrence of Garnets with the Trap of New Haven, Connecticut. American Journal of Science: s. 3: 14: 215.

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