![[View larger image]](#5154899276173586657_i_326-lg.jpg.id-760661789912705730.wrap-0.html.html){kind=link}
The whole of the southerly portion of the main island is composed of a highly weathered rock which has thin veinlets of serpentine cutting through it. Running in a north and south direction, and in places dragged and folded and cutting this formation, there is a dyke, which stands up prominently from the main country rock. About 30 yards to the east the rock is cut by a series of irregular interlacing narrow dykes having the appearance of old concrete. The ground mass is hard and to the eye amorphous. It contains rounded pebbles and possibly shell remains.
Towards the centre of the main island the rock formation changes abruptly to a compact glassy green rock, probably a peridotite. It has developed a jointing, and but for the conglomerate forms the remainder of the island and possibly the other islands as well, because the country rock on the north island is of a similar nature.
Along the inside of the central basin at two points there occurs a conglomerate—pebbles ranging from 3 inches in diameter to a fraction of an inch cemented in a matrix.
Towards the north end there is a fault which crosses the island in a N.W. and S.E. direction, and parallel to which there is a dark, rusty dyke.
In two or three places on the main island, one of which is near this fault, there are small pot holes. There was a rounded boulder in each, and probably, as the sea comes swirling in at high tide, a rotary motion is given to the boulder and the pot hole develops.
The general formation of the islands might be described as a stock of glassy peridotite which has risen from the bed of the ocean and of which only the highest points are now visible.
Saõ Miguel Açores, St. Vincent (Cape Verde), Ascension and St. Helena
The above islands were called at and examined, but as the geology has already been described by others who had more time at their disposal, no new light was thrown on them. The visits, however, were valuable in that they will enable the writer to compare the conditions existing at these places with the seldom visited islands already above described.