- GeoRef, Copyright 2004, American Geological Institute. Reference includes data from Bibliography and Index of Geology Exclusive of North America, Geological Society of America, Boulder, CO, United States
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.