Besides the volcanoes we have already described, there are many others situated in mid-ocean far from any continent. A brief description will be given of a few of these.
All the three great central oceans, the Pacific, the Atlantic, and the Indian, contain numerous volcanic islands, some of which rise many thousands of feet above the general level.
We will begin with a description of some of the more important volcanic islands of the Pacific. It was first pointed out by Kotzebue, and afterwards by Darwin, that all the islands of the Pacific Ocean can be divided into two great classes, the _high islands_ and the _low islands_. All the high islands are of volcanic origin, while the low islands are of coral formation. It is the opinion of Dana, who has made a careful study of coral formations, especially in the Pacific, that in all probability even the low islands of the Pacific were originally volcanic, and that the deposits of coral had been made along their shores after their volcanoes had become extinct.
The islands of the Pacific, like the shores of the continents and most of their mountain ranges, extend in two great lines of trend, or general direction, which intersect each other nearly at right angles. These lines extend from the southeast to the northwest, and from the northeast to the southwest respectively, those extending in a general direction from southeast to northwest being the most common in the Pacific.
Now, perhaps, the greatest number of the earth's volcanoes are arranged along fissures, or cracks in the earth's crust. The craters are situated along the cracks, the openings being kept clear at the crater, and gradually closing elsewhere, probably by pressure. In other words, most of the volcanoes follow one another along more or less straight lines. For example, in the western part of South America they follow the Andes Mountains. A similar arrangement exists in the volcanoes of Central America, Mexico, and the United States. Now, this is especially true of mid-ocean volcanoes of the Pacific which lie along lines extending from southeast to northwest, or from northeast to southwest, though mainly along the former.
Some of the volcanic islands of the Pacific have already been described or referred to, as, for example, the Aleutian Islands, which stretch in a curved line from the southwestern extremity of the peninsula of Alaska to Kamtschatka on the coast of Asia. We have already described the island of Hawaii, the great volcanoes of the Sandwich Islands chain, and besides these there are in the North Pacific the Ladrone Islands, lying east of the Philippines.