402 
THE VOYAGE OF H.M.S. CHALLENGER. 
faces mjm answers within several minutes to the angle of the same faces in phillipsite. 
Here then we are dealing with the fundamental forms of this mineral, and, at least in 
the case of the smallest of these microliths, with non-twinned crystals, which have not 
hitherto been pointed out in specimens of this species found in fissures and geodes of 
volcanic rocks. In spite of their extreme tenuity, the faces c and b can each give good 
reriections, thus showing that these crystals are not lamellar, as might be supyjosed at 
first sight, but that they possess a develoj)ment almost equal for these faces. The 
attemyjts to determine their optical y^roperties, difficult even in the case of large crystals, 
have not given any definite results. The optical properties of phillipsite are very variable, 
and, as mth the majority of minerals belonging to the group of zeolites, the tints of 
chromatic polarisation are of low order, and in this case the difficulty is increased owing 
to the great absorption of light by the optic apparatus when studying between crossed 
nicols with high magnifying powers. In fine, the angle of extinction of phillipsite is 
relatively small, and as the edges are only a few hundredths of a millimetre in length it 
is difficult to measure this angle under the microscope. When the crystals are larger, 
there may sometimes be observed at the two extremities four lozenge-shaped faces repos- 
ing upon the edge oo 5oo /oP, having then the aspect of orthorhombic prisms terminated 
by the faces of a pyramid. AVhat has been said as to the determination of the faces 
shows that we are dealing in this case with one of the ordinary twins of phillipsite, the 
plane of twinning and the plane of composition being the face oP. It may be added that 
this twinned form has not up to the present time been observed in an isolated condition, 
exceyit in the case of small crystals of phillipsite from Plombieres, where Des Cloizeaux 
lias found forms identical with those here indicated. 
The small crystals are seen to pass through all the transitions of size to the larger 
simple or twinned individuals, which show a tendency to group themselves irregularly or 
according to a crystallographic law. Even the smallest microliths that 
yiass off with the first decantation are superposed, grown together, and 
interlaced. In certain cases the groups are regular ; they are crossed 
twins, recalling perfectly the well-known twin form of harmotone and 
of })liilliyisite. The annexed woodcut (Fig. 36) reyiresents one of these 
twinned crystals from the South Pacific, Station 276, 2350 fathoms; 
Kiff. 36. — CronHed Twin 
sution ”*^ 276 ,'"'^ 2 . 3 M it is from tliis station that all the figured specimens of zeolites from 
the Pacific have been selected. 
fathoms, South 1‘ociflc. 
This cruciform twinning is reyieated 
so frequently and is so characteristic that it might almost of itself serve to identify 
these little cr}*stals as l»elonging to the one or the other of these zeolitic species. 
Although the twinning is not rare, the crystals arc more frequently observed forming 
irregular groups, as shown in PI. XXII. fig. 4, where the.se crystals are grouped as they 
aj»j>ear after isolation from the mud by decantation. The grouped microliths are covered 
by a coating of inar)gane.se and iron, which is generally arranged around the centre of the 
