Gravity data indicate that there is a regular relationship between crustal structure, crustal density (composition), and surface elevation. Earthquake and surface seismic refraction and reflection evidence as to the composition and structure of the earth's crust have not yielded a simple, unambiguous relationship to the surface elevation. The velocity dispersion of earthquake surface waves, on the other hand, indicates variations in the thickness and composition of the crust that are in general accord with the variations in surface elevation and the Bouguer gravity anomalies. Why seismic refraction measurements have not agreed everywhere with gravity and surface wave indications of crustal structure appears to be a result of masking of crustal layering. On the basis of the slope of the curve that describes the relationship between the seismic depth to the Mohorovicic discontinuity and Bouguer gravity anomalies, the density difference between the crust and the mantle appears to decrease as the thickness of the crust increases. On the assumption that the mantle has a constant mean density of 3.32 gm/cc, the mean crustal density would appear to increase from a minimum value of 2.86 gm/cc in the oceans to about 3.08 gm/cc beneath the high plateaus and mountains. If the mean crustal density is essentially constant, the effective density of the mantle must decrease by a comparable amount. The existence of a low‐density zone in the upper part of the mantle, as suggested by the velocity dispersion of very long period Rayleigh waves, would explain the relationships observed. Isostatic relationships suggest that the mean density of the continental crust is essentially constant (2.85 gm/cc to 2.88 gm/cc). These values imply that a basaltic layer is present everywhere. That there is possibly an increase in mean crustal density as the crust thickens is suggested by U.S.S.R. seismic studies in Central Asia. These show that the intermediate (basaltic) layer is usually thicker beneath areas of uplift. Although the origin of the basaltic layer can only be surmised, its general inhomogeneity, as indicated by variations of seismic velocity from 6.4 to 7.3 km/sec, and its varying thickness suggest that it may be a zone of phase transformation within the underlying mantle rock. Despite the lack of homogencity in the crust, it appears possible that empirical relationships may be used to predict approximate crustal thickness from the regional Bouguer gravity anomalies or from surface elevations with a reliability approaching that for seismic measurements.

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The U. S. Geological Survey has just announced the publication of the Bouguer Gravity Anomaly Map of the United States. This map, fourteen years in the making was prepared by the writer as Chairman of the AGU Special Committee for the Geophysical and Geological Study of the Continents. The active cooperation of many individuals and groups throughout the country was essential since the number of gravity observations, running into hundreds of thousands of measurements distributed on an eight‐ to fifteen‐mile spacing over much of the country, represents an undertaking beyond the capacity of a single individual or group. The five major sources of data were oil companies, government agencies, private exploration companies, mining companies, and universities. As much of the data was of a proprietary nature, the sponsorship of the AGU was a critical factor in obtaining it from company sources, since management policy does not favor the release of information that has cost the company large sums of money, especially to an individual. Similarly, the role of the AGU cannot be overemphasized in obtaining the active cooperation of government agencies and universities in releasing data prior to publication.

Observations with a LaCoste and Romberg geodetic gravimeter having a very low nearly linear drift rate, a high reading precision, and a world wide range were made at approximately three hundred sites in order to check and extend the gravity control network in North America. The sites occupied were mostly at former gravimeter bases located at airports, harbors, universities, and pendulum stations. The instrument was calibrated against the North American standardization range of pendulum measurements from Paso de Cortes, Mexico, to Fairbanks, Alaska, using the weighted mean values of the observations established with the U. S. Coast and Geodetic Survey, Cambridge University (England), and Gulf‐University of Wisconsin pendulum equipment. A statistical evaluation of the precision of the network based on the reoccupations at 40 major control stations gives an estimated standard deviation of 0.08 mgal. The airport network of bases previously reported by Woollard (1958) that was established with high range Worden gravity meters was found to require a systematic correction of 0.3 mgal per 1,000 mgal change because of the difference in calibration standard used. The adjusted values for the forty airport stations reoccupied agree on the average to 0.2 mgal with the results of this study. The reoccupations of the old pendulum observation sites of the U. S. Coast and Goedetic Survey suggest that much of this network is in error by over 3 mgals. Descriptions of sites occupied and the principal facts for position, elevation, observed gravity, and free‐air and Bouguer anomalies are presented.