The sill is 15 feet at its thickest part and consists of two layers of basalt adjacent to the country rock, two hybrid zones on the inner margins, and a center felsite zone up to 8 inches thick. It was intruded into Jurassic sandstone in two closely spaced stages. The first was the intrusion of the basalt, with slight effects on the country rock; the second was the intrusion of the felsite,, probably from a higher level in the crust, along a line of weakness in the middle of the sill. Chemical analyses of the basalt and felsite, norms, and spectrographic data accompany the petrographic descriptions.

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