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University of California. 



[Vol. 3. 



crystal grains or fern-like skeleton crystals. In the more crys- 

 talline basalts it seems to be in rod-like forms, giving lath-shaped 

 sections. The iron ore is believed to be generally or always 

 magnetite, more or less titaniferous. The characteristic form 

 and cleavage of ilmenite were nowhere observed. 



The apatite has the usual form of slender hexagonal prisms, 

 often greatly extended in the direction of the vertical axis. Its 

 prominence varies widely in the different specimens owing to the 

 fact, whose demonstration may be more logically presented in 

 the discussion of structure, that apatite is in these rocks one of 

 the later secretions from the magma. 



The secondary material, which to 1 a great extent replaces 

 olivine, all seems to belong to the species iddingsite, but presents 

 a wide range of variation in structure and color. The color 

 ranges from reddish brown to deep grass-green. The double 

 refraction appears to be somewhat lower for the latter variety. 

 The iddingsite occurs in these three ways : as pseudomorphs after 

 olivine, as a filling in cavities, and in minute vein-like streaks. 

 The replacement of olivine begins on the outer surface and along 

 the cracks of the original mineral and works inward. The final 

 result of the process carried to its completion may be a 

 pseudomorph of felted scales or fibres, often showing a mesh- 

 structure like that of true serpentine in altered peridotites. On 

 the other hand, the iddingsite may form a true paramorph, 

 behaving optically as a unit, as it does in the typical occurrence 

 described by Professor Lawson.* When this mineral occurs as 

 a filling of cavities, the first layer is generally made up of fibres 

 perpendicular to the walls of the cavity, while the centre is filled 

 in with a fine, irregular, felted aggregate. 



In certain vesicular facies of the basalt, good specimens of 

 the zeolites, natrolite and analcite are found. The former 

 occurs in the usual radiate- acicular structure, often forming 

 spherulitic masses an inch in diameter. The analcite occurs as 

 a filling in small cavities, but in many larger cavities it takes 

 the form of beautiful clear trapezohedral crystals implanted in 

 the walls. 



We now turn to the consideration of the structural variations 



* Loc. cit. • ' 



