San Rafael Swell

Dykes, sills and plugs cover an area of more than 200 km2 in the area of the San Rafael Swell. Dykes vary up to 3 m in thickness and sills up to about 35 m. The dykes are usually uniform in composition but the sills are nearly all composite, an outer chilled contact rock being succeeded by a mafic zone up to 6 m thick, which in turn passes into a central, leucocratic rock which appears to be intruded later and produces sharp or gradational contacts. The mafic rock may be equigranular or porphyritic with phenocrysts of olivine, biotite and augite, zoned to green rims, and labradorite in a ground mass of analcime, thomsonite, natrolite, sodic amphibole, biotite, a little augite and accessories. Modes and chemical analyses are available.