stripes
Nepheline syenite and phonolite form flat-lying sheets, dykes and a small stock which are usually intruded into Eocene rocks in the central part of the Oregon Coast Range. The most extensive developments are the 65-95 m thick sills that cap Table and Cannibal Mountains, which were probably originally continuous and extended over 500 km2 (Snavely and Wagner, 1961, p. 158). At Blodgett Peak nepheline syenite forms a small stock. The nepheline syenites contain euhedral phenocrysts of albite (An10) in a groundmass of nepheline, analcime, aegirine-augite, olivine, riebeckite and opaques. At Table Mountain the albite phenocrysts have been extensively altered to sericite and clay minerals. Chemical analyses are available (Snavely and Wagner, 1961, Table 344. 1). A sample of nepheline syenite from Table Mountain has been developed as a USGS rock standard and described in detail by Snavely et al. (1976). Pb isotopic data on a sample from Table Mountain are given by Tatsumoto and Snavely (1969).
MACLEOD, N.S. and SNAVELY, P.D. 1973. Volcanic and intrusive rocks of the central part of the Oregon Coast range. Bulletin, Oregon Department of Geology and Mineral Industries, 77: 47-74.
SNAVELY, P.D. and WAGNER, H.C. 1961. Differentiated gabbroic sills and associated alkalic rocks in the central part of the Oregon Coast Range, Oregon. Professional Paper, United States Geological Survey, 424D: 156-61.
SNAVELY, P.D., MACLEOD, N.S., FLANAGAN, F.J., BERMAN, S., NEIMAN, H.G. and BASTRON, H. 1976. Nepheline syenite, STM-1, from Table Mountain, Oregon. Professional Paper, United States Geological Survey, 840: 7-10.
TATSUMATO, M. and SNAVELY, P.D. 1969. Isotopic composition of lead in rocks of the Coast Range, Oregon and Washington. Journal of Geophysical Research, 74: 1087-100