interpolated from the contour map and averaged to give a value for the 1x1° square. 

 For areas such as the Caribbean the above procedure was not practical since the 

 gravity values change too rapidly. In such areas the contours as well as the 

 locations of spot values were digitized and computer programs were written to 

 interpolate the gravity values at the center of 10x10' squares. These values were 

 averaged to obtain free-air gravity averages over 1x1° squares. 



Figure 6 gives the free-air gravity values averaged over 1x1° squares. The 

 values over ocean areas were obtained by the methods outlined above. In ocean 

 areas, as for example north of 50°, the number of available gravity measurements 

 were insufficient to allow us to draw contours. In such areas we have used the 

 unadjusted free-air values to obtain averages over 5x5° squares. The description 

 of the gravity field in these areas is considered poor. 



We have not been able to utilize data from some of the most recent cruises 

 whose tracks are shown in Figure 1. Hence the averaged values in Figure 6 over 

 ocean covered areas are subject to revision. The 5x5° values in particular might 

 be revised drastically on the basis of newer data. 



The 1x1° free-air gravity values of Figure 6 are contoured at an interval of 

 20 mgal in Figure 7. Contours are dotted in the 5x5° square areas. 



3. LAND GRAVITY DATA 



We have obtained free-air averages on land from published data. Values in 

 southeast Cuba were averaged from Shurbet and Worzel (1957) . The values in Mexico 

 and in other parts of Cuba were obtained from a Russian Bouguer gravity map of 

 North America published in 1968 (conversion from Bouguer to free-air anomalies 

 were made by using regional elevations) . After having made the calculations in 

 this study we became aware of the compilation of Woollard et al . (1969) for Mexico. 

 Using the poorer Russian data in Mexico constitutes the biggest source of error in 

 this study. 



23-11 



