McMurdo Volcanic Group, northern ultramafic inclusions occur around the base of the volcano. Mount Overlord has a 1-kilometer-wide caldera and a very Victoria Land youthful appearance. Surprisingly, three potassium-argon (K- Ar) age measurements give a mean date of about 7 million years (Armstrong 1978). Exposures in the caldera walls show that the PHILIP R. KYLE upper part of the volcano is composed of alternating trachyan- desite and trachyte flows and associated pyroclastic rocks. One Department of Geoscience of the final eruptions carried a large number of plutonic New Mexico Institute of Mining and Technology xenoliths to the surface. Some of these are derived from the Socorro, New Mexico 87801 local granitic basement; however, the great majority are be- lieved to be cognate and to consist predominantly of essexites, JOHN A. GAMBLE syenites, and nepheline syenites. A detailed study of the petrol- ogy and geochemistry of Mount Overlord is in progress. Department of Geology Parasite Cone, about 8 kilometers northwest of Mount Over- Victoria University lord, is composed of hyaloclastite which apparently formed Wellington, New Zealand during a subglacial eruption prior to the downcutting of the Astronaut Glacier and Aviator Glacier valleys. Radiometric dat- WILLIAM C. MCINTOSH ing of Parasite Cone may aid in defining former ice levels in the area, especially for the Evans Névé, and may provide a max- New Mexico Bureau of Mines and Mineral Resources imum age for downcutting of the Aviator Glacier drainage. Socorro, New Mexico 87801 Hyaloclastites are also common along Aviator Glacier, par- ticularly northwest of Eldridge Bluff and Co-pilot Glacier. Un- MARK NOLL fortunately, most of these hyaloclastites are inaccessible for sampling. Department of Geoscience A very young, small, basaltic vent is located on the south side New Mexico Institute of Mining and Technology of the Cosmonaut Glacier, about 2 kilometers upstream from Socorro, New Mexico 87801 Aviator Glacier. During a period of higher ice level, lava flows poured down a steep granite wall into Cosmonaut Glacier, creating a chaotic deposit of intermixed till, hyaloclastite, and As part of the 1981-82 northern Victoria Land field camp, we basalt. The deposit caused a slight diversion in the flow of the examined the Late Cenozoic alkali volcanic rocks of the McMur- glacier. do Volcanic Group, from 17 December 1981 to 14 January 1982. This research was supported by National Science Foundation Investigations centered mainly on Mount Overlord and grant DPP 80-20002. vicinity; however, reconnaissance studies also were made in the lower Mariner Glacier area and at the Pleiades (Kyle 1982), Cosmonaut Glacier, Vulcan Hills, and Aviator Glacier. It is ap- References parent that McMurdo Volcanic Group rocks are more extensive than preliminary reconnaissance geologic mapping had indi- Armstrong, R. L. 1978. K-Ar dating: Late Cenozoic McMurdo Volcanic cated (Gair et al. 1969). The area between the lower Mariner Group and dry valley glacial history, Victoria Land, Antarctica. New Glacier and the Borchgrevink Glacier is probably all volcanic, Zealand Journal of Geology and Geophysics, 21, 685-698. mainly basaltic hyaloclastites similar to those from the Hallett Gair, H. S., Sturm, A., Carryer, S. J. , and Grindley, G. W. 1969. The volcanic province described by Hamilton (1972). The volcanics geology of northern Victoria Land (Folio 12, Plate 12). In V. C. Bushnell (Ed.), extend from sea level to the summit of Mount Phillips at the Geologic maps of Antarctica, 1:100,000, Antarctic maps folio series. New York: American Geographical Society. south end of the Malta Plateau, a thickness in excess of 3,000 Hamilton, W. 1972. The Hallett volcanic province, Antarctica. (Professional meters. Paper 456-13). Reston, Va.: U.S. Geological Survey. Mount Overlord is a 3,396-meter-high stratovolcano com- Kyle, P. R. 1982. Volcanic geology of The Pleiades, northern Victoria posed predominantly of differentiated alkali rocks ranging from Land, Antarctica. In C. Craddock (Ed.), Antarctic geoscience. Madison: trachyandesite to trachyte. Younger basaltic cones containing University of Wisconsin Press.

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