Appendix

The digital data and map of this open-file report are 
available from the following U.S. Geological Survey URL:  

http://geopubs.wr.usgs.gov/open-file/of99-435/

Data Contents

The digital dataset consists of two files (lm_100k.iso and 
lm.prop) containing 1,573 gravity stations and 290 density 
and susceptibility measurements.  The file, lm_100k.iso, 
contains the principal facts of the gravity stations, with 
one point coded per line.  The format of the data is 
described below.  Each gravity station has a station name, 
location (latitude and longitude, NAD27 projection), 
elevation, and an observed gravity reading.   The data are on 
the IGSN71 datum and the reference ellipsoid is the Geodetic 
Reference System 1967 (GRS67).  The free-air gravity 
anomalies were calculated using standard formulas (Telford 
and others, 1976).  The Bouguer, curvature, and terrain 
corrections were applied to the free-air anomaly at each 
station to determine the complete Bouguer gravity anomalies 
at a reduction density of 2.67 g/cc.  An isostatic correction 
was then applied to remove the long-wavelength effect of deep 
crustal and/or upper mantle masses that isostatically support 
regional topography.

The file, lm.prop, contains the density and magnetic 
susceptibility information for each sample measured, with one 
measurement coded per line.  Densities were meassured using 
an electronic precision balance and susceptibilities were 
measured using a KT-5 susceptibility meter.  The densities 
are accurate to 0.01 grams per cubic centimeter.


                 EXPLANATION OF PRINCIPAL FACT FORMAT


Item                                     Explanation

STATION NAME (a8) ............An alphanumeric combination of up to 8
                                characters used for station identification

LAT (f3.0,f6.3)...............Latitude in degrees and minutes, to 0.01
                                minute

LON (f4.0,f6.3)...............Longitude in degrees and minutes, to 0.01
                                minute

ELEV (f8.2)...................Elevation, to 0.1 ft

OG (f10.3)....................Observed gravity, to 0.01 mGal

FAA (f9.3) ...................Free-air anomaly to 0.01 mGal

SBA (f8.3)....................Simple Bouguer anomaly to 0.01 mGal

ITC (f7.3)....................Inner terrain correction for a density of
                                2.67 g/cc, to 0.01 mGal, followed by a
                                letter denoting the extent of the correction.
                                "Z" indicates computer terrain correction 
                                from the station out to 166.7 km with inner
                                terrain correction out to Hayford D zone.

TC (f7.3).....................Total terrain correction from the station to
                                166.7 km for a density of 2.67 g/cc, to 0.01
                                mGal.

TC CODE (a1)..................Letter denoting the extent of the correction, 
                                according to the Hayford-Bowie template (e.g.
                                'D' means 590 meters).

CBA (f8.3)....................Complete Bouguer anomaly reduced for a
                                density of 2.67 g/cc, to 0.01 mGal.

ISO (f8.3)....................Isostatic residual anomaly values assuming an
                                Airy model for isostatic compensation of
                                topographic loads.  This model assumes a
                                a crustal thickness of 25 km, a topographic
                                load density of 2.67 g/cc, and a density 
                                contrast across the base of the model crust
                                of 0.4 g/cc.

ISO-COR (f8.3)................Isostatic residual anomaly values corrected 
                                for the effect of Lake Mead

                 Example of format for gravity file

51636676 36 12.16 114 29.15  2480.0 979579.35   -23.18 -107.77   3.13   5.96 
D -102.71   -3.14   -2.91

STATION NAME:     51636676
LAT:              36 degrees 12.16 minutes North
LON:              114 degrees 29.15 minutes West
ELEV:             2480.0 feet
OG:               979579.35 mGal
FAA:              -23.18 mGal
SBA:              -107.77 mGal
ITC:              3.13 mGal
TC:               5.96 mGal
TC CODE:          D (590 m)
CBA:              -102.71 mGal
ISO:              -3.14 mGal
ISO-COR:          -2.91 mGal


         Example of format for physical property file

W97-118B   36.0092  114.0325  2.51 2.24 2.06 0.04  Limestone (Tertiary) 

STATION NAME:              W97-118B
LATITUDE:                  36.0092 degrees North
LONGITUDE:                 114.0325 degrees West
GRAIN DENSITY:             2.51 grams per cubic centimeter
DRY BULK DENSITY:          2.24 grams per cubic centimeter
SATURATED BULK DENSITY:    2.06 grams per cubic centimeter
MAGNETIC SUSCEPTIBILITY:   0.04 x 10-3 cgs units
ROCK TYPE:                 Limestone (Tertiary)



Telford, W.M., Geldart, L.O., Sheriff, R.E., and Keyes, D.A., 
1976, Applied Geophysics:  New York, Cambridge University 
Press, 960 p.