THE VOYAGE OF H.M.S. CHALLENGER. 
basaltic series (see PL XVII. fig. 3), but tlie most characteristic forms, and probably also 
the most frequent, are extremely thin rhombic tables. As by the disintegration of the 
palagonite these little crystals of plagioclase are sometimes detached from the matrix 
and found in a free state in the clay, we have been able to study them in an isolated 
condition, and to determine with exactitude their form and nature. It may be 
mentioned in passing that these rhombic tables of felspar play an important role among 
the minerals present in the deposits of those regions where the basic glasses are 
abundant. When these tables embedded in the vitreous fragments are examined by 
means of transparent slides, they are seen to be so thin that the vitreous material 
still covers them on all sides, and at first sight one might regard their contours as traces 
of regular fractures of the mass in which they are enclosed ; this aspect is represented 
in PI. XVII. fig. 2, where near the base of the figure two of these little crystals can be 
seen in a mass of red altered glass, their contours being almost entirely hidden by the 
enclosing matrix. In othe^ preparations they are better developed, and on account of 
their thickness can be readily recognised, as, for instance, in the vitreous altered 
fragment represented in PI. XIX. fig. 1. The lapilli here represented is remarkable for 
the abundance of these rhombic lamellae, the fundamental vitreous mass being decomposed 
into palagonite ; in certain points the manganese has infiltrated and masked the palagonitic 
matter by its deep brown tint. On this background the unaltered sections stand out in 
relief, with the exception of some parts of their surface still covered by a thin vitreous 
layer or coloured by man^nese. 
These plagioclase crystals show a colourless transparent mass, in which the following 
forms may be observed : — The most frequent forms are flat tabular crystals, with the 
clinoi)inakoid especially developed. Individuals of the columnar type, elongated in the 
direction of the edge P/M, are rare. These tabular crystals consist essentially of a 
combination of the clinopinakoid with P and x, more rarely with P, u, and y, and 
occasionally x and y appear together. In the first case the crystals have the form of a 
rhomb, in the second cjise they are elongated through the predominance of either x or P. 
Tlie dimensions of those crystals which were examined and measured lie between 0‘61 
mm. broad and 1 mm. long as maximum, and 0‘015 mm. broad and 0’042 mm. long as 
minimum. Tlie extinction of the plagioclase is negative. Its value was found to vary 
between 22° and 32° on the clinopinakoid, and between 8° and 16° on the basal plane. 
The average values of many measurements made on good crystals are as follows : — 
24° 12', 25° 6', and 29° 6' on the clinopinakoid ; 10° 42' on the one side, and 10° 18' on 
the other side, of the twinning line, as this is shown on the basal plane. Polysynthetic 
individuals, made up of repeated twins on the albite plan, were very rarely observed. 
The felspar, in its optical jiropcrtics, is thus seen to be between labradorite and bytownite. 
The twin growths are particularly frequent, and interesting on account of the structure 
of the individual.s. In addition to those of the albite type, others were observed in 
