30 KOTO : NOTES ON THE GEOLOGY 



same chemical composition as tliat of analcime, and tlie physical 

 properties observed give no hinderance to the assumption that 

 this component actually is that miueral. He thinks the analcime 

 is primary, having been formed from the magma, containing 

 water and much soda, under pressure with considerable rapidity. 

 From what has been stated before, I have also, to all ap- 

 pearances, the primary analcime in the interspaces of the com- 

 ponents in the Basalt from Gio-ô, and the radiating bundles of 

 a strongly biréfringent natrolite are formed secondarily from the 

 analcime through a molecular rearrangement. Both components 

 make their appearance with the dodecahedric networks {PL II, 

 Fig. 5) of the skeleton magnetite which occupies the other portion 

 of the slides. 



THE IRON ORES. 



Both ilmenite and magnetite are present, and they usually 

 belong to a single generation, and indeed the product of the 

 eftusive period, as the iron ores were not found enclosed in the 

 olivine of the intratelluric crystallisation. Both ores, especially 

 the ilmentite, have crystallised later than plagioclase, but slightly 

 prior to, or contemporaneous with, the monoclinic pyroxene. 

 The ilmenite and magnetite are, under the microscope, not easy 

 to be distinguished, as every petrographer will agree, if crystal 

 forms are not serviceable for their diagnosis. 



The ilmenite is, however, tabular and needle-shaped in sec- 

 tion in the Basalt with a strong lustre and a violet tinge, when 

 seen by reflected light, on the flanks corresponding to the 

 thickness of slide. The laths are slender, appearing merely as 

 lines, and cross several crystals of felspar and augite, mean- 



