102 Alfred Harker—Rocks of the ‘‘ Beagle”? Collection. 
touched at Porto Praya, on the south coast of Santiago, the largest of 
the Cape Verd group. Darwin utilised his time in exploring the 
geology of the port and the neighbouring parts of the island, and 
collected about 150 specimens. We shall refer to the more interesting 
of these in the order in which they are mentioned in the first chapter 
of ‘‘ Volcanic Islands.”’ Of some of them thin slices have been made, 
and the number of the slides, in the Sedgwick Museum cabinet, will 
be given in brackets. 
The lowest rocks on the coast near Porto Praya and on Quail Island, 
underlying the white Tertiary limestone, are highly basic, non- 
felspathic lavas. The fresher specimens show a very dark and 
compact ground-mass, with crystals of yellow olivine and black 
augite, which are usually very abundant. Three examples which 
have been sliced illustrate three different types, and probably represent 
fairly the whole group. The first [4704] is a limburgite. Olivine is 
the dominant mineral, in perfectly fresh well-shaped crystals. 
Augite is also abundant, in good crystals with some tendency to 
stellate grouping. It is very pale in the slice, and often shows fine 
lamellar twinning. There are also a few little octahedra of magnetite, 
sometimes enclosed in the olivine. These minerals constitute the 
greater part of the rock, but there is in addition an abundant glassy 
base of deep brown colour, enclosing very numerous little rectangular 
gratings of magnetite. 
The second type [4705] shows the same minerals, except that the 
olivine is largely replaced by serpentine and carbonates. ‘The augite 
is strongly zoned. ‘The ground-mass consists of a second generation of 
augite and magnetite, with slender needles of apatite and a clear 
isotropic base, which is quite colourless. This is evidently an 
example of the second variety of limburgite distinguished by Doelter 
(op. cit., pp. 184-137) as having a colourless instead of a brown glass. 
The true nature of the colourless base is, however, doubtful, and we 
shall recur to the subject below. 
The third type [4706] differs from the foregoing in that olivine is 
scarcely represented. The phenocrysts are of a pale yellowish-brown 
augite, zoned and often twinned, with magnetite. The ground-mass 
consists, as in the last specimen, of abundant little idiomorphic augites, 
magnetite, many needles of apatite, and a colourless isotropic base, 
‘which is here in rather larger amount. This rock evidently belongs 
to the ‘pyroxenites’ of Doelter, which Rosenbusch more conveniently 
terms ‘augitites,’ the former name being preoccupied by a very 
different group of rocks. As in the limburgites, Doelter (op. cit., 
pp. 187-143) distinguishes two kinds, one having a brown glassy base 
and the other a colourless base, assumed to be also glassy. 
The dykes mentioned by Darwin (p. 3) as intersecting these lavas 
‘seem to be of related types, but the only one sliced [4713] shows 
some differences. Augite preponderates over olivine among the 
porphyritic crystals. The ground-mass, in addition to augite and 
magnetite, contains little slender crystals of felspar, some with 
twinning. It is noteworthy that most of these crystals give low 
1 The common English spelling is here adopted. Darwin writes the name 
‘St. Jago,’ and Doelter uses the Portuguese form ‘§. Thiago.’ 
