104 Alfred Harker—Rocks of the “ Beagle ” Collection. 
again of monchiquitic affinities. One fresh example has been sliced 
[4711]. It is a dark-grey rock of compact texture, enclosing 
abundant olivine, which is partly red from incipient change. 
Different stages of the process of alteration are shown by other 
specimens, the final product being the ‘‘dark jasper-red earthy 
mineral,” with ‘‘indistinct cleavage,” described by Darwin in 
a footnote. It is one of the variable and imperfectly-known alteration 
products of ferriferous olivine to which iddingsite and several allied 
substances belong. The slice shows the olivine here to be only 
slightly altered. As usual in these rocks, its crystals are sharply 
formed, as are those of the less plentiful augite. The latter mineral 
is very pale brown, with the zonary banding well marked between 
crossed nicols and sometimes assuming the hour-glass arrangement. 
There are also grains of black iron-ore and a few small flakes of 
biotite.. The ground-mass is composed of numerous little crystals of 
augite and magnetite, with interstitial clear analcime and the usual 
apatite needles. The rock may be named a monchiquite. Another 
sliced specimen from the neighbourhood of the same hill [4712] is 
closely similar to the preceding, but the colourless interstitial matter 
shows some difference. For the most part it is isotropic, and may 
probably be set down as analcime, though the cleavage is not so 
evident as in the other slice. There are, however, patches, not 
distinguishable from the rest in natural light, which are birefringent, 
and are probably nepheline. It may be recalled that this mineral is 
found occasionally in the original monchiquites of Brazil. 
The .‘‘inland hills of more ancient volcanic rocks’? have not 
furnished many specimens to Darwin’s collection, and none of these 
have been sliced. Fresh examples from the ‘“‘ upper strata”’ are dark 
compact rocks enclosing abundant little grains of olivine, and they 
may be allied generally to the liimburgites and monchiquites, though 
in one case.a few slender crystals of felspar are just discernible. The 
specimens from the ‘basal strata”? are not fresh enough to be 
diagnosed, and are of close texture without visible crystals. They 
‘have a yellowish-white colour, with ferruginous staining, and it is 
possible that they are, as Darwin supposed, of trachytic nature, 
though Doelter found no true trachytes in the Cape Verd Isles. 
Several specimens are described in the Catalogue as ‘‘ various 
crystalline rocks forming more central part of island,” and are stated 
to come from north-west of Porto Praya, without closer specification. 
They evidently belong to the same group of lavas as those found in 
the coast. district, and may be from some of the same flows. One 
example sliced [4707] is a imburgite like the one described above 
from Quail Island [4704], except that the brown glass is here more 
abundant than in the former case, and encloses only a few minute 
crystals of magnetite instead of the abundant skeleton growths. 
Another specimen [4708] is not very different. The porphyritic 
elements are the same as before, except that the olivine is now 
replaced by carbonates. In the ground-mass, besides the little 
crystals of augite, there are others of felspar, giving moderate 
extinction-angles, and the interstitial base is the usual deep brown 
glass of the limburgites. 
