22 ARCTIC BASALT PLATEAU 
they are partly changed to thomsonite. The mafic constituents pre- 
dominate, but they are much decomposed: the olivine phenocrysts 
are nearly almost replaced by a homogeneous mineral with strong 
pleochroism (pale yellow to greenish brown) and high double re- 
fraction, biaxial negative with medium axial angle and a distinct 
cleavage normal to the acute bissectrix. These pseudomorphs are 
closely suggestive of the produce received by Thaddéeff! on heating 
olivine rich in ferrous silicate. The few unaltered remnants of fresh 
olivine allowed no definite determination of the ferrous silicate con- 
tent by means of refractive indices. The pale brown augite forms 
by dimensions the proportional transition to the groundmass; it is 
much altered by late magmatic influence, partly to chlorite and 
partly to a mineral of the serpentine group, and builds also up 
glomerophyric aggregates, intermixed with magnetite grains and 
ilmenite needles. Apatite needles are seen chiefly as inclusions in 
the plagioclase phenocrysts, but also in residual interstices. A strange 
' appearence offer the great pseudophenocrysts of thomsonite corres- 
ponding to the zeolite patches mentioned above: they are surrounded 
by a narrow envelop of analcite and small quantities of natrolite 
needles, and although formed later than and at the expense of the 
enveloping minerals, as shown on sections normal to the zone of | 
prisma, they imitate closely veritable phenocrysts. — The small 
residual interstices are filled out by small quantities of analcite, and 
locally also natrolite needles are seen, but their quantity is unim- 
portant. No strong evidences of glassy residue were detected. 
Since the mineral composition is being altered (hydrothermally?) 
and no rock fragment free from zeolite patches for analytical in- 
vestigation is obtainable, the measurement of the actual mineral 
composition by the Rosiwal method was not undertaken; never- 
theless an analysis of a specimen poor in zeolite was executed by 
Dr. N. Sahlbom, and gave the following results: 
1 K. Thaddéeff, Die Olivingruppe. Zeitschr. f. Krystallographic 26 1896. p. 60. 
