Alfred Harker—Rocks of the ‘‘ Beagle’ Collection. 105 
_ A specimen from the precipice surrounding the village of S. Domingo 
(‘‘ Volcanic Islands,” footnote on p. 20) shows the usual abundant 
erystals of olivine set in a dark compact ground-mass, but a thin slice 
[4714]. brings out certain differences. The olivine crystals are 
transformed, marginally or sometimes totally, into a deep red-brown 
mineral, having a high birefringence comparable with that of the olivine 
but with oblique extinction. There are also a few small flakes of 
biotite in the rock. The general ground-mass shows abundant minute 
crystals of augite, with some of magnetite, in a colourless base, 
enclosing very abundant needles of apatite. ‘The base is in part 
isotropic, doubtless analcime, but largely of a birefringent mineral, 
which seems to be felspar rather than nepheline. Much of it has the 
appearance of sanidine. In places, however, the interstitial material 
has segregated out into relatively large clear patches, and these consist 
partly of analcime, partly of a lamellated plagioclase with low 
extinction-angles. 
It appears from the foregoing notes that the prevalent types of 
lavas in the southern part of this island are of highly basic or ultra- 
basic composition, and belong principally to the limburgite-monchiquite 
group, though including some aberrant varieties. There remain to 
be noticed the phonolites, which may perhaps be regarded as the 
leucocratic complements of the preceding. Darwin remarks that, 
among the trachytic-looking rocks which make the lower parts of the 
flat-topped hills inland from Porto Praya he found in three places 
‘smooth conical hills of phonolite, abounding with fine crystals of 
glassy felspar, and with needles of hornblende’’ (‘‘ Volcanic Islands,” 
pp. 19, 20. In the Catalogue two of the specimens are described as 
from ‘paps’ peeping up among the ‘‘ various crystalline rocks”’ 
(limburgites, as we have seen) north-west of Porto Praya, and the 
third from ‘‘ pap-like hills composed of vitreous felspar’”’ north-east of 
Porto Praya. Darwin’s account seems to imply, though not very 
clearly, that their relations are intrusive, but Doelter terms these 
little hills ‘Kuppen.’ The specimens show fresh crystals of sanidine, 
up to 3 inch in length, in a compact ground-mass of lighter or 
darker shades of grey, with the characteristic lustre imparted by 
abundant nepheline. While belonging to the phonolite family, they 
exhibit considerable variety of characters. The three specimens sliced 
illustrate as many different types, and they differ also from one 
described by Doelter! from the same neighbourhood. 
In the first type of phonolite [4710] the porphyritic elements are 
sanidine and egirine, with some crystals apparently of altered 
nepheline and a few octahedra of magnetite. The egirine is green, 
with fairly marked pleochroism, and much of it occurs as aggregates 
of little erystals making pseudomorphs after larger crystals. ‘The 
ground-mass, constituting the greater part of the rock, is composed of 
very abundant little crystals of nepheline with sanidine. 
In the second type [4709] the porphyritic elements are more 
abundant, and include a greater variety of minerals. In addition to 
the large sanidines there are well-shaped crystals of nepheline, fairly 
1 Op. cit., pp. 88-91, with chemical analyses. The word ‘nephelinarmen’ at the 
beginning of the description should be ‘ nephelinreichen.’ 
