Chlorite arises from biotite, augite and hornblende.
In Minahassa, at the northern extremity, there is a large area of tuffs and agglomerates consisting chiefly of augite andesite, and in this area there are many recent volcanic cones.
The larger ones polarize light, have angular outlines like those of crystals, and may even show twinning and definite optical properties by which they can be identified as belonging to felspar, augite or some other rock-forming mineral.
The cones themselves are composed largely of acid andesites, but many of the lavas are augite andesites and basalts.
Some are black, very thin and curved like threads or hairs (trichites); often a group of these is seated on a small crystal of augite or magnetite and spreads outwards on all sides.