Page:A sketch of the physical structure of Australia.djvu/23

11 however, shew themselves at different portions of its course, sometimes forming whole mountain groups, and there is probably no very considerable portion of the chain without the exhibition of granitic rocks, either in its central ridges or on some of its flanking or lateral ranges. Gneiss and mica slate do not seem to be any where abundantly exhibited, although they not unfrequently occur, together with considerable masses of clay—slate and other rocks more or less metamorphic. Great masses of porphyry and greenstone are often associated with these, especially in the lateral ranges.

Upon these rocks lies a large palæozoic formation, which is principally composed of very thick masses of sandstone interstratified occasionally with beds or groups of shale and some beds of limestone. In this palæozoic formation, perhaps only in a particular part of it, beds of good coal are found. Masses of porphyry, greenstone, basalt, and other igneous rocks, are associated with the palæozoic formation, often cutting through, dislocating, and altering various parts of it, but sometimes seemingly supporting it as if existing before its production.

Tertiary rocks again are found reposing at various places on all the above-mentioned rocks, up to a certain height above the sea, and these are likewise associated with still more recent igneous rocks, which sometimes retain all the characters of veritable subaerial lavas.