230 C. N. Fenner — Crystallization of a Basaltic Magma 



Fig. 10. 



Resorption of Olivine in the Watehung Basalt. 



In hand specimens, both of the vitreous and the aphanitic 

 varieties of the Watehung basalt, small, dark-green spherulitic 

 bodies can often be seen. They appear in many of the thin 

 sections and are found to be built up of concentric shells of 

 lighter and darker material of chloritic or serpeiitinons nature. 

 A typical example is shown in slide No. 105 (tig. 10). 



The diameter of the spherulite is about 0"2 mm , but larger 

 ones are not uncommon. The central portion is composed of 

 comparatively large blades of radial 

 chlorite, decidedly birefringent. The 

 outer shells are very feebly polarizing 

 and appear to be made up of minute 

 scales of chlorite mingled with isotropic 

 glass. Beyond the border of the spheru- 

 lite the normal vitrophyric structure is 

 found, consisting of small crystals and 

 grains of feldspar and diopside and 

 brown microlitic tufts, set in the paste 

 of glass. In this case resorption and 

 alteration have progressed so far that 

 there is almost nothing suggestive of 

 the manner in which the spherulite originated, but in other 

 cases the evidence is plain. Outlines of some original min- 

 eral are left, to whose resorption by the magma the spheru- 

 lites are attributable. None of the mineral' itself remains, 



Fig. 10. Chloritic nod 

 iile in vitrophyr (Slide No 

 105). 



Fig. 11. 



Fig. 12. 



Fig. 11. Partially resorbed olivine, subsequently altered. (Slide No. 

 105.) 



Fig. 12. Separation of oxide of iron along cracks in olivine crystals 

 which have been replaced by secondary products. 



the crystal outlines being filled with chlorite, calcite and 

 other alteration products. These retain certain features, how- 

 ever, which point strongly to olivine. 



