During the past thirty years the development of improved methods of analysis, especially optical spectrography, x-ray spectrography, and colorimetric methods, has resulted in a wealth of new data on the abundances of many elements. Nevertheless, present estimates differ greatly in reliability; some, such as the estimate for bismuth, are based on very few actual determinations, whereas others, such as that for gallium, are based on analyses of many samples.

This paper reviews the estimates of abundance for the elements bismuth, germanium, gallium, and yttrium, taken as examples to illustrate the wide variation in reliability. For gallium and yttrium, a study has been made of spectrographic analyses by four laboratories of more than 600 chemically analyzed rocks from many areas of the world. Comparison shows that these furnish valuable data on abundance, although attention must be directed to regional variation. The comparison also shows the need for continued interlaboratory standardization.

The estimate for bismuth is based on so few determinations that no appraisal of its accuracy can be made. The estimate for germanium is based on very few determinations but is almost certainly of the right order of magnitude, though perhaps slightly too high. The estimates for gallium and yttrium, based on many determinations, are almost certainly of the right order of magnitude, though that for gallium may be slightly low and that for yttrium may be substantially low.