TERTIARY VOLCANIC ROCKS-LATITE-PHONOLITE. 
73 
optical properties—that the mineral is analcite. In some cases grains of calcite 
are embedded in the analcite and probably owe their existence to the presence of 
lime in the original feldspar. 
Another decomposition product of the feldspars, perhaps both orthoclase and 
plagioclase, appears as a foliated or fibrous mass usually oriented parallel to some 
face of the feldspar phenocryst. It is colorless, has a refractive index somewhat 
higher than that of Canada balsam, and gives parallel extinction and interference 
colors in yellows and reds of the first order in sections of usual thickness. These 
properties and the mode of occurrence correspond to alunite. But a test of a rock 
in which it was abundant failed to give a reaction for sulphuric acid, and alunite 
is hence excluded. The mineral corresponds in some ways to hydrargillite, but 
its identity is not established. Sericite results to some extent from the plagioclase 
also in the vicinity of mineral veins. 
Of the groundmass feldspars, orthoclase is by far the most abundant. Albite 
specimens are present in a few. The habit of these youngest feldspars is similar to 
that in the phonolites. The lath forms are most common, but small flakes of 
iregular outline are often observed. 
Pyroxene .—This mineral group is an important constituent of the iatite- 
phonolites. Phenocrysts up to 1 cm. or more in cross section, frequently as 
anhedrons, a but more often of definite crystal form are, because of their habit, 
and the composition of the magma, necessarily augite. They are usually of pale 
green color, slightly pleochroic to light greenish yellow. Prismatic cleavage is 
fairly well developed, c: c is always over 40° and frequently rises to 58° or 60° 
in which case the colors are deeper, the pleochroism more marked, and the mineral 
becomes aegirine-augite. The interference colors are rather high. Arrangement 
in zones of slightly different composition is a rather common feature, and twinning 
parallel to the orthopinacoid is not unusual. Large individuals sometimes inclose 
grains of feldspar, and in certain instances an interpenetration with hornblende 
is well shown. 
Pyroxene is generally abundant in the groundmass. In some specimens it 
occurs in little bundles and sheaves of green needles and grains, much as in the 
phonolites. In a few others it forms narrow laths with intersertal distribution. 
The most common occurrence is as microlites and small rounded grains rather 
evenly scattered among the groundmass feldspars. 
The most common alteration is to carbonates, black iron ore, and chlorite; 
occasionally quartz also may be seen. Serpentine is sometimes a product, epidote 
forms at times, and not infrequently a mass of light-green secondary mica (described 
on p. 77) replaces and retains the form of the p}"roxene individual. 
Hornblende and biotite .—Hornblende is not unfailingly present, but usually occurs 
in individuals of fairly good form and moderate size. Occasionally it becomes quite 
prominent. It is strongly pleochroic in browns and yellows and shows absorption 
as follows: c > b > "*, c and b often being not very different. Cleavage is distinct as 
usual. Twinning is not uncommon. The maximum extinction angle in clino- 
diagonal sections is 26°. Intergrowth with pyroxene is common and sometimes the 
latter is completely surrounded by a fairly even rim of hornblende. Corrosion has 
a Pirsson, L. V., Bull. Geol. Soc. America, vol. 7, 1896, p. 492. 
