In 1884 I spent about a fortnight investigating the geological structure of these islands, and intended to have to a certain extent completed my studies at that time. Cholera was then raging at Campobasso on the mainland, and the islanders, not comprehending the curious habits of a travelling geologist, supposed I had been sent by the Government to spread the “Cholera Powder,” and requested me to leave the islands by the next steamer. This I naturally for safety's sake did, and opportunities for returning to the locality not having again presented themselves to me, and lest I should forget some of the interesting observations I then made, I think it desirable without further delay to record what I then saw. This is all the more important since my own observations differ considerably from those of the few other observers of the geology of these islands, or include facts which they do not mention in their writings.

I Will now put forward some details, for it seems necessary that some proofs of what has been maintained in general should be given, and I shall therefore quote some examples of the intimate connexion between earth-structure and terrestrial magnetism. No diagrams could indicate the relations better than Locke's Magnetic Sections across the Hudson, which were taken at three different points of the Palisades, Snake Hill, Fort Lee and Patterson (see Plate XV.). The dip and intensity were in each case determined at a number of points along a line perpendicular to the range of cliffs, and the curves rise very suddenly to maxima just above the edges of the cliffs, which are composed of columnar diorite. This diorite is widely distributed and lies between masses of Triassic sandstone; the dip is slight and towards the west.

The felsites of the south-east of Ireland are shown on the maps, of the Geological Survey to extend over considerable areas in counties Wicklow, Wexford and Waterford. Like the Welsh felsites they are contemporaneous with Lower Silurian (Ordovician) strata, and were probably erupted on an old sea-bottom. They are accompanied by abundant deposits of tuffs and breccias, the component fragments of which consist mainly of felsite.

The origin of the Triassic rocks of Britain is a question that has excited from time to time much interest and varied speculation. The entire absence of fossils in the Lower Trias or Bunter Sandstone has led the majority of geological reasoners to look to causes other than marine action for an explanation of its characteristic features. The late Mr. Godwin-Austen was of opinion that the whole of the Triassic rocks were laid down in freshwater lakes which, passing through the brackish stage, become finally saturated with saline matter through evaporation exceeding the inflow of fresh water. It has been felt, however, by other geologists, that this theory, while accounting fairly well for the upper deposits of the Triassic age, does not fit in with the phenomena of current bedding, the presence and distribution of the numerous quartzite and other well-rounded pebbles, and the comparative absence of marl-beds, which distinguish the Lower Trias or Bunter Sandstones.