The Abrolhos Islands.
INFLUENCE OF OCEAN CURRENTS.
THE ZOOLOGICAL PROVINCE OF AUSTRALIA.
Although there are few places on the Australian coast where one can spend a more enjoyable summer holiday than the Abrolhos, both on account of the free and “simple life” they afford, and the great historic interest they possess as being the scene of the final catastrophe which befel the Dutch attempt to colonise Australia in 1629 under the unfortunate Pelsart, the islands themselves are of peculiar interest, and have given rise to much speculation as to their origin.
They are all composed entirely of dead coral, here and there partially covered by wind-blown sand, and for the most part not rising more than ten or twelve feet above the water, while many of them do not attain an elevation of more than three or four feet. They are situated on the 29th parallel of south latitude, and are probably further removed from the Equator than any other coral islands in the world, while the indigenous vegetation, such as it is, belongs to a latitude well within the tropics. It is a curious speculation therefore, how these islands came to exist in their present position.
When we “lay to” on our recent trip our boat drifted considerably northward, while closer to Geraldton the northerly current is still more defined. These northerly flowing waters are the cold waters of the Southern Ocean travelling into the equatorial regions, where they are warmed and supply the current which passes westerly and southerly through the Indian Ocean, attaining its greatest velocity as it passes through the Mozambique Channel, thence round the Cape of Good Hope, and north-westerly through the Carribean Sea to form the Gulf stream, which takes a north-easterly course to impart their warm, genial climate to the British islands.
The equatorial current of the Pacific is broken up and delayed in its course as it passes through the Indian Archipelago and Sunda Islands, and its waters become warmer than those of any other oceanic waters on the globe, while the current itself is split up and diverted into innumerable directions, which make navigation extremely intricate and dangerous. It finally emerges from this intricate maze of islands at about latitude 15 south and longitude 115 east, where portion of its waters join the easterly current to Mozambique, and the remainder flows about south by west, being kept from the Australian shores by the stream of cool water from the Southern Ocean, which hugs the Westralian coast line till it reaches well into the tropics, and merges into the warmer water of Oceania between Sharks Bay and Java, as a lower current. A glance at the map of the world will show exactly how the islands of the Indian Archipelago would operate in forming the intricacies of the currents along the north-west coast.
Owing to the rotation of the earth being from west to east, the tendency of all currents in the ocean is to flow westward, unless some local cause deflects them from that course. The most notable example of this deflection is the Gulf stream, while the next most important is the Pacific current in its ramifications through the islands of the Indian Archipelago.
Polar waters always flow towards the equator, still partaking, however, of the westerly direction unless deflected by local causes; and, being cold, flow beneath the warmer currents, which have been expanded by equatorial heat, until, being warmed, they gradually rise and mingle with the equatorial currents, which, in addition to their westerly motion, also flow towards the Poles, till, becoming cooled, they sink and merge with the Polar waters, thus maintaining a continued circulation.