Petrology of the Lavas. 335 



ing the titaniferous augites seen in certain nephelinites. Like 

 those, it is pleochroic and shows a dispersion of the bisectrices. 

 The augites of the groundmass are similar, though not markedly 

 zoned, rather evenly developed in the prism zone and relatively 

 long; columnar on the vertical axis. The sections are, there- 

 fore, lath-shaped. While sometimes unaltered, they are often 

 in part, or wholly, replaced by granular masses of carbonates. 

 In several cases it has been observed that where the augites of 

 the groundmass project into or lie in areas of analcite the 

 common variety described is replaced by segirite or segirite- 

 augite. Similar cases have been noted by Cross* in the phono- 

 lite of Cripple Creek, and by the writer in analcite-basalt from 

 the Little Belt Mts.,f and is one indication of the original 

 nature of, at least, some of the analcite. 



Biotite occurs only as a small occasional flake in the ground- 

 mass. 



Olivine, in an altered condition, has been found in only one 

 case by the writer, embedded in the section. A few isolated 

 fragments, some altered to greenish serpentine, others very 

 fresh, have been found in the powders, but the amount is so 

 small in comparison with augite that it must be considered a 

 rather rare phenocryst in the Bermuda lavas. The decom- 

 position products indicate a variety rich in iron. 



Iron ore is well distributed, usually in small crystals rather 

 evenly scattered through the groundmass. The square outlines 

 indicate the octahedron, and that it is probably magnetite. 

 The most notable fact about the iron ore is the way in which, 

 in the solid lavas below 700 feet, it appears unchanged, even 

 in groundmasses much altered, while in the zone above that 

 level it has largely gone into limonite. 



Apcclite occurs in slender colorless needles in the ground- 

 masses and occasionally in larger stouter prisms. On account 

 of the suspected presence of melilite, whose optical properties 

 are very similar to those of apatite (melilite o, 1*634; e, 1*629 ; 

 o-e, 0*005 ; colorless, uniaxial, cleavage poor ; apatite, o, 1*638 ; 

 e, 1*634; o-e, 004; colorless, uniaxial,- cleavage poor) a 

 chemical test was made for phosphoric acid, since it had not been 

 determined in the quantitative analysis. The finely ground rock 

 was boiled in dilute nitric acid, filtered, and the nitrate evap- 

 orated to dryness. The residue was taken up in nitric acid, 

 diluted, the insoluble silica filtered off, and in the filtrate 

 molybdic acid solution gave an abundant test for phosphoric 

 acid. This confirms the identity of the grains and needles. 



Titanite was observed in several of the samples in isolated 



* Geology of the Cripple Creek District, Colo., 16th Ann. Rep. TJ. S. 

 Geol. Snrv., Pt. II, p. 35, 1895. 

 f Twentieth Ann. Rep. of the U. S. Geological Surv., Pt. Ill, p. 546, 1900. 



