Page:South African Geology - Schwarz - 1912.djvu/175

 the rift valleys of the west of Cape Colony. The vents sometimes run together as an open fissure out of which gushed lava. Many of the vents are exposed in the seaward side of the range, showing that the greater portion of the original pile of lavas, including the central mass, has been denuded away. This is due to the fact that the lavas were piled up across the rivers that originally flowed from the main watershed to the Indian Ocean. The rivers on the land side of the volcanoes were thus turned back, collected into the Orange River, and the waters were made to flow across the whole breadth of Africa to the Atlantic. The rivers on the seaward side, on the other hand, had only a short course to the sea and therefore having very great fall had enormously greater erosive power than the rivers in the land side of the volcanic range with their extended course, barred in addition by the granite of Augrabies Falls. Hence the crest of the volcanic range has crept away from the sea.

It is not usual for volcanoes to emit lavas entirely without ash. The Hawaiian volcanoes do so; Mauna Loa being a pile of lavas 30,000 ft. high measured from the sea floor. There is remarkably little ash in the volcanoes of the Drakensberg; a little is preserved in those vents which never reached the mature stage of lava emission, but beds of ash occur very sparingly. Some of the vents may still be seen with the lava issuing from the opening just as it formed in Jurassic times.

The Stormberg Formation is well represented in Australia, where it follows the marine Permo-carboniferous with the Coal Measures which there take the place of the South African freshwater Karroo Beds.