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University of California. 



[Vol. 



distinguished by the absence of olivine. They will be described, 

 for the sake of ready comparison, in this section, but under a 

 separate heading-. 



Olivine Basalts. — The differences to be observed in the various 

 specimens of the olivine basalts are chiefly structural. Differences 

 in conditions of cooling have naturally given rise to the various 

 degrees of crystallization, so that we have, on the one hand, 

 intersertal basalts with a large proportion of glassy base, and, 

 on the other, holocrystalline rocks with typical ophitic or 

 granulitic structure. Between these extremes we may trace 

 complete gradations. In view of their mineralogical likeness and 

 structural diversity, the most convenient method of describing 

 this group of rocks will be to consider first the characters of the 

 constituent minerals, and afterward the structural variations. 



The primary minerals, in the usual order of abundance, are 

 basic plagioclase, augite, olivine, iron ore, and apatite. All of 

 these may be considered essential except the last, which is an 

 accessory occurring in variable amount. Glass forms an essential 

 part of a large majority of specimens. The secondary minerals 

 are the serpentinoid alteration products of olivine, and analcite 

 and natrolite occurring in amygdaloidal cavities. 



The feldspars in every specimen examined seem divisible into 

 two classes on the basis of size; yet, although this distinction has 

 always forced itself upon the observation of the writer, there is 

 usually no such contrast, either in size or form, as exists between 

 the phenocrysts and the groundmass laths of a typical andesite 

 or trachyte. The large feldspars of these basalts are greatly 

 elongated in the direction of the braehyaxis, and more or less 

 flattened on the brachypinacoid. The average relative develop- 

 ment in the direction of the three principal axes may be roughly 

 expressed by the proportion a- b- c '-' 6- V- 2. They are distin- 

 guished from the smaller crystals by greater regularity of outline. 

 The feldspars of the small size are generally characterized by 

 skeleton forms and intricate outlines. As inclusions in all the 

 feldspars there are negative crystals of glass clouded with 

 magnetite dust. Less common are magnetite crystals and rounded 

 grains of augite; these occur in the large feldspars, in which 

 they have commonly a zonal arrangement. In polarized light, 



