H. S. Washington — Basalts of Kula. 121 



These theories are, it is admitted, but probable, not certain, 

 and for the further discussion of the subject the reader must 

 be referred to the original dissertation. 



Other minerals. — The other mineral constituents need not 

 detain us long. The olivine is very sparingly present, and 

 offers no special features of interest. Two cases of inclusions 

 of black trichites, in groups, were noticed — a rare occurrence 

 in basaltic olivine. The magnetite of the groundmass is 

 always in very small grains, rarely reaching a diameter of 

 0-l mm . The most interesting feature of its occurrence is that 

 in the least glassy Kulaites the magnetite grains are very 

 abundant and the glass basis perfectly colorless, while as the 

 rock becomes more glassy the quantity of magnetite decreases 

 and the glass assumes a brown hue, till finally in the pure 

 tachylytes no magnetite is to be seen and the glass is of a clear 

 dark chocolate brown color. This seems to show that in these 

 rocks the magnetite is among, if not quite, the last of the 

 minerals to crystallize out. Its rarity as an inclusion points 

 the same way. It may be noted that Judd* observed exactly 

 the opposite state of affairs in the Tertiary gabbros and basalts 

 of western Scotland. The leucite offers no special features of 

 interest. That apatite is present is shown by the presence of 

 P 2 O s in the analyses, but crystals of it were not certainly seen, 

 except a few large ones, which are certainly derived from 

 enclosures of foreign rock. 



General description of the rocks. — All the basaltic rocks of 

 the Kula district are very similar to one another in composition 

 — chemical and mineralogical, — as well as in the habit of their 

 mineral components, and form a good, though small, example 

 of a so-called " petrographical province." Structurally they 

 may be divided into normal, hyalopilitic, semi-vitreous, and 

 tachylytic basalts, these subdivisions shading off into one 

 another. 



To the first of these belong the specimens of the second 

 period streams — light gray, rough fine grained rocks, of aver- 

 age sp. gr. 2*756, with porphyritic augite and olivine. Under 

 the microscope they show a groundmass of glass and lepto- 

 morphic plagioclase grains containing many microlites and 

 magnetite, with small porphyritical augite and olivine crystals, 

 and numerous hornblende crystals, which have all, without 

 exception, undergone alteration, generally completely to a mass 

 of augite and opacite. Rounded forms are quite common. 



The third period basalts are more glassy, and a hyalopilitic 

 structure, of quite typical development, is common, the micro- 

 lites being plagioclase laths, augite needles, and magnetite 



*Q. J. Geol. Soc, 1S8G, 79. 



