J. B. Ferguson — Molybdenum in Rocks. 401 



special study of the Hawaiian lavas, kindly undertook the 

 examination of the slides for me, and his notes on them are 

 given below :* 



A. Lava dipped from the Pit of Halemaumau. 



"Black, porous, few white and yellowish mineral grains. 

 Luster brilliant vitreous. Pores round with glossy walls." 



" Microscope : Glass predominant, yellow-brown, very fresh. 

 Has few minute bubbles ; some pores have narrow zones of 

 ferritic glass about them ; some are in pure glass. Fuzzy 

 streaks or trains, of ferritic substances, more or less curved, 

 connect some pores, Labradorite, augite and olivine are all 

 prominent in distinct crystals. Augite and labradorite tend 

 to form groups. Olivine occurs isolated and some are more or 

 less markedly skeleton crystals. All are quite fresh. Magnet- 

 ite is very rare and is mostly included in silicate grains." 



B. Lava from Kilauea floor. 



" A dull black finely porous lava with megascopically 

 abundant feldspar and augite, and less olivine." 



" Microscope : There is much labradorite and augite with 

 tendency to form groups of several grains of each. Olivine is 

 less abundant, but occurs in larger isolated crystals and is very 

 fresh. Main mass of rock is greatly obscured by ferritic 

 material, but there is rude development of plumose bundles or 

 radiate aggregates of silicates. These forms consist of labrador- 

 ite needles with attached augite grains, the ferritic substance 

 obscuring their character. Both rocks are quite normal basalts 

 of the Kilauea type." 



The molybdenum was determined in all cases on 5 gram 

 samples, the proceduref advocated by Hillebrand being closely 

 followed. With reference to the occurrence of molybdenum 

 in the Hawaii lavas, it must be remarked that in the two 

 basalts, A and B, the molybdenum tests were made in dupli- 

 cate, and decidedly strong traces were found in each case. 

 As especial precautions were taken in the duplicates to avoid 

 contamination, the result can not be attributed to accidental 

 impurity. As the amount of the trachyte obsidian was 

 limited, only one test was made. The amount found was 

 rather less than in A or B. The amount in A exceeded that 

 in B, and an attempt to weigh it gave the percentage as ap- 

 proximately 0*01. The negative tests obtained in the other 



* Attention may be called to the fact that the short description of these 

 lavas by Merwin (Day and Shepherd, Bull. Geol. Soc. Amer., xxiv, 573-606, 

 1913) was made on the powders, and not on thin sections, which may serve 

 to explain apparent discrepancies between the two descriptions. 



fBull. U. S. Geol. Survey, No. 422, p. 150, 1910. 



