Geomagnetic time-variations observed at several sites on the island of Hawaii are analysed for the effects of island bathymetry as well as for the inductive response of the deeper mantle. The data are generally consistent with the deep conductivity profile derived using lower frequency, electromagnetic data from the Island of Oahu. Hawaii data fit better if that model is modified to give the upper 200 km of the mantle a lower conductivity of 0.02 S/m compared to 0.1 S/m for Oahu. The data are represented by a complex, frequencydependent function of location, Tu, relating the vertical variation Z to a component U of the horizontal variation (T, = Z/U). The direction of U is nearly frequency independent at each site but is different for each site. Below a frequency of about 30 cycles per day, the functions, T,, at any two sites are found to be related by a real constant. This suggests that the deeper conductivity structure is the same beneath each site. This result is consistent with quasi-static induction in a non-uniformly conducting thin sheet above a stratified conductivity structure. The response of such a model can be written as Tu = A q , where 4 is a quasi-uniform, complex, frequency-response function characterizing the effect of the deep conductivity and A is a spatially dependent parameter parameterizing the effect of variable conductivity in the thin sheet. The parameter A may be estimated by fitting observational estimates of T, to models of deep conductivity structure.Present address: Pacific Marine Environmental Laboratory, NOAA, Seattle, Washington 96 105.

Petrophysical data on dry bulk density, porosity and magnetic susceptibility from 52 rock specimens from southeast Arizona and southwest New Mexico are tabulated and summarized by major geologic map units.The rock specimens were collected within the Silver City 1° x 2° quadrangle, primarily in parts of the Pinaleno, Peloncillo, Gila, Summit and Big Burro Mountains. All but ten of the samples are from Tertiary volcanic units.iii Density, porosity, and magnetic susceptibility of rocks from the Silver City 1° x 2° quadrangle, Arizona and New Mexico

Petrophysical data on 364 rock specimens from southwest Arizona are tabulated and summarized by major rock suites for that part of southwest Arizona covered by the Ajo 1° x 2° quadrangle and the Papago Indian Reservation. Data for 202 of these specimens are new and previously unreported. The tabulation also contains data from theses by B. A. Hargan and B. T. May, University of Arizona. iii Density, porosity, and magnetic properties of rock specimens from southwestern Arizona

At the request of the BLM, over 100 square miles (260 square kilometers) in the Comudas Mountains of southern Otero County, New Mexico, were evaluated for their potential for undiscovered mineral resources. There are three BLM-designated study areas in the Cornudas Mountains: Alamo Mountain, Wind Mountain, and Comudas Mountain Areas of Critical Environmental Concern (ACEC). All three ACECs include scenic peaks comprised mainly of Tertiary alkaline igneous rocks that rise above the surrounding gently rolling topography. This report incorporates geological, geochemical, and geophysical data collected in 1996 by the authors as well as compilation of existing data. The Comudas Mountains are at the eastern edge of the Basin and Range Province and within the Tertiary Trans-Pecos magmatic belt. The area is underlain by easily eroded Permian carbonate rocks and Cretaceous clastic rocks that are intruded by more resistant alkaline plugs and sills. Permian rocks exposed are, from oldest to youngest, the Hueco, Yeso, and San Andres Formations. Outcrops of Lower Cretaceous rocks are restricted to areas near intrusions, and much of the exposed Cretaceous occurs as blocks in landslide deposits along the edges of intrusions. Intrusion of the plugs and sills caused folding and warping of the Paleozoic and Cretaceous rocks; fold axes are predominantly east-trending. Surface folding, as well as geophysical data, indicate unexposed intrusive rock underlying the Comudas Mountains. The Comudas Mountains have been explored for nepheline syenite for industrial use and a resource is identified at Wind Mountain for industrial grade glass. No other exposed intrusion has potential for industrial grade glass. The area has been explored for metallic deposits associated with alkaline igneous rocks-uranium, beryllium, rare-earth elements, niobium, and gold and silver but has low potential for these deposit types. The geochemical environmental consequences of future mining would have minimal impact on the water chemistry of subsurface waters. Nepheline syenite in the Comudas Mountains, if mined, has a limited buffering capacity and contains little or no pyrite or other sulfide minerals that generate acid and degrade the water. Previous Investigations U.S. Bureau of Mines (USBM) geologists, at the request of the BLM, independently conducted an examination of identifiable mineral occurrences in the Wind and Alamo Mountains ACECs in 1992 and 1993 (Korzeb and Kness, 1994) and in the Chess Draw area northwest of Wind Mountain in 1992 (Schreiner, 1994). The only potential mineral resource identified in these evaluations was industrial grade nepheline syenite that underlies Wind Mountain. Previous investigations by the U.S. Geological Survey in this area are restricted to reconnaissance mapping by G.O. Bachman and associates during the compilation of the Geologic Map of New Mexico (Dane and Bachman, 1965).

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