402 J. B. Ferguson — Molybdenum in Rocks. 



rocks, using the same reagents as with the Hawaiian rocks, 

 speak for the purity of the reagents employed. 



From these results, combined with those of Hillebrand, it 

 would therefore appear that while molybdenum favors the 

 more siliceous rocks and is generally rare or absent in the less 

 siliceous or more femic ones, yet in certain regions it may be 

 present even in the latter. The evidence at hand, so far as it 

 goes, also indicates that the presence of molybdenum is not 

 correlated with high soda or potash content, at least in the 

 petrographic provinces the rocks of which were examined. 

 Its presence in detectable amounts would seem to be influenced 

 by regional characters rather than by general chemical ones, 

 except for the pronounced tendency toward association with 

 highly siliceous rocks first noted by Hillebrand. The unex- 

 pected results obtained, however, indicate the advisability of 

 occasionally searching for molybdenum, which is readily detect- 

 able if present, even though the analytical processes are some- 

 what tedious. 



Geophysical Laboratory, 



Carnegie Institution of Washington, 

 March, 1914. 



