Geologically considered, the country may be divided into three regions - a central, and the largest, comprising the whole width of the Aravalli system, formed of very old sub-metamorphic and gneissic rocks; an eastern region, with sharply defined boundary, along which the most ancient formations are abruptly replaced by the great basin of the Vindhyan strata, or are overlaid by the still more extensive spread of the Deccan trap, forming the plateau of Malwa; and a western region, of very ill-defined margin, in which, besides some rocks of undetermined age, it is more or less known or suspected that Tertiary and Secondary strata stretch across from Sind, beneath the sands of the desert, towards the flanks of the Aravallis.
Nevertheless, under some of these flows remains of plants and insects of species now living in the islands have been found - a proof that the formation as well as the denudation of the country is, geologically speaking, recent.
Flora.-The pastoral wealth of Uruguay, as of the neighbouring Argentine Republic, is due to the fertilizing constitutents of "pampa mud," geologically associated with gigantic antediluvian animals, whose fossil remains are abundant.
Of all the Asiatic ranges the Himalayan is, geologically, the best known; and the evidence which it affords shows clearly that the folds to which it owes its elevation were produced by an overthrust from the north.
Geologically, spiders date from the Carboniferous Period, Arthrolycosa and others from the coal beds of Europe and North America being closely allied to the existing genus Liphistius.
