Equally indecisive is the further exploration as to evidence for the opinion held by other naturalists that the endemic species of the different islands have resulted from subsidences, through volcanic action, which have reduced one large island mass into a number of islets, wherein the separated species became differentiated during their isolation.
Past elevations of land, however (and doubtless equally great subsidences) have taken place in South America since the Eocene, and the conclusion that extensive areas of land have subsided in the Indian Ocean has long been based on a somewhat similar distribution of giant tortoises in the Mascarene region.
In the Moscow basin, which was a broad gulf of the Carboniferous sea, coal appears as isolated inconstant seams amidst littoral deposits, the formation of which was favoured by frequent minor subsidences of the seacoast.
The geological sequence of events appears to have been the following: - After the deposition of the Eocene (or Oligocene) limestone - which reposes upon a floor of basalts and trachytes - basalts and basic tuffs were ejected, over which, during a period of very slow depression, orbitoidal limestones of Miocene age - which seem to make up the great mass of the island - were deposited; then elapsed a long period of rest, during which the atoll condition existed and the guano deposit was formed; from then down to the present time there has succeeded a series of sea-level subsidences, resulting in the formation of the terraces and the accummulation of the detritus now seen on the first inland cliff, the old submarine slope of the island.
The same inconvenience is felt in the environs of Nancy, and a similar one produces on a larger scale the sinking and subsidences at Winsford and Northwich.
A parliamentary commission was appointed in 1881 to investigate the causes of the disastrous subsidences which are constantly taking place in all the salt districts, and the provision of a remedy.
In its narrow and irregular streets many of the houses are strongly bolted to keep them secure from the subsidences which result not infrequently from the pumping of brine.