File:The Quarterly journal of the Geological Society of London (13960399383).jpg

COLLECTED IN MADAGASCAR BY REV. R. BARON. 355

phyritic crystals. The presence of hornblende in the original magma

is marked by pseudomorphs of iron-ore.

Andesitic lavas occur at Andranonatoha and on a hill in the

volcanic region to the west of Lake Itasy. They are of a darker

colour than the trachytes, namely, greyish brown. They contain

striated felspar, which generally gives lath-shaped sections, but also

occurs in large irregular masses, imbedded in a ground-mass com-

posed of prisms of yellowish-green augite, granules of magnetite,

and microlites of felspar. A glassy base does not appear to be

present, unless it be as extremely thin films between the felspars of

the ground-mass. Large porphyritic crystals of hornblende floated

in the molten magma when it was first erupted: but they have

been converted into granules of iron-ore; and only small fragments

of the original hornblende-substance remain to mark its former

presence.

To summarize briefly, we have found the older crystalline series

of Madagascar, as represented by Mr. Baron's collection, to consist

of foliated rocks (which are described as granitite-gneiss and tonalite-

gneiss), and rocks in which no trace of foliation can be detected,

comprising granite, olivine-norite, pyroxene-granulite, and pyroxenite.

The majority of the granites are of the granitite type, but true granite

(i.e. granite with tivo micas) also occurs. The basic members of the

unfoliated series are interesting on account of their striking mineral

combinations. The clear aspect and bright colouring of their con-

stituent grains often make them objects of surpassing beauty when

viewed under the microscope. Of deeper interest is the fact that

these basic types, so well known in other areas of crystalline schists,

— in Saxony, Brittany, Scandinavia, Scotland, and on the Hudson

River — constitute in Madagascar, as at Kilamanjaro on the adjacent

mainland, so large a part of the ancient floor on which the sedimentary

rocks were laid down and through which the volcanic lavas were

erupted.

The volcanic rocks consist mainly of basaltic types, only a few

specimens of trachyte and andesite being represented in Mr. Baron's

collection. The basalts vary, as regards composition, with respect

to the presence or absence of quartz, olivine, porphyritic and micro-

litic hornblende, and biotite. One curious type contains idio-

morphic crystals of hornblende as a constituent of the ground-

mass. A felspar-free variety of magma-basalt is also represented.

This rock contains only a small quantity of olivine, and is therefore

intermediate in composition between the limburgite of Kosenbusch