The Australian continent has undergone great changes during the past geological ages, and most probably has been connected in remote times with part of Asia, and not unlikely with South America by some now submerged land. But whatever the connection may have been in the very distant past, it has been shut off from the larger northern land masses at so remote a period that the higher forms of mammals have not found their way to it, as in Africa and South America. Great changes have taken place in the continent itself. It is supposed that, at one time, in what is called the cretaceous or chalk age, a great sea spread from the north right across from what is now the Gulf of Carpentaria, covering immense tracts of level plain country in the interior of Australia, including Western Queensland, and part of New South Wales, so that the western half of the continent was separated from the eastern at least in the northern parts. Gradually the land rose and great lakes were formed in the interior, especially in the region of Lake Eyre, and a growth of vegetation sprang up of a more luxuriant type than is to be found now in those western parts, otherwise the enormous animals, such as the giant diprotodon, huge extinct kangaroos, birds larger than the moa, as well as crocodiles and turtles, could never have found sustenance to multiply in such numbers as their fossil remains testify they did in nearly every part of central Australia, and in the interior of North Queensland. In this sea, which washed the base of the mountains on the west, was deposited the sandy formation which has become the level inland plains. From some cause so far unknown, the land became desiccated, the lakes lost their freshness, and became great salt pans, the vegetation and the animals dependent on it became extinct, until a dry and arid region was produced, with a river system that fails to reach the sea, but becomes absorbed in the great sandy interior. The smaller types of marsupials of a hardier nature and capable of removing to greater distances for food, maintained their existence, while the giants of a similar race have left only their bones embedded in the drift to testify to the mighty changes that Nature has wrought out in the past ages. Fossil diprotodons of gigantic size and struthious birds rivalling in stature the New Zealand moa, are found in Central Australia. At Lake Callabonna in the great salt Lake Eyre basin, there are hundreds of fossil skeletons of these animals, many of which have been removed to the Adelaide Museum. In that locality they are found most frequently on the surface of the dry salt lake, and have been preserved by a natural coating of carbonate of lime; the bones are found at various depths.