52 



deposit would be caused. The lower beds would consist of the 

 finer clays and sediments, and the uj^per beds nearest the streams 

 source be the coarser and more sandy. Waters containing lime 

 in solution deposit it readily when relieved of pressure, and also 

 its iron when flowing among materials which furnish carbonic 

 acid. But silica which is held in solution by water is only 

 precipitated on complete evaporation, and therefore a country 

 which receives such waters must eventually be covered with a 

 floor of chalcedony, or if previously a sandy formation, the upper 

 surface must be changed to a porcelainised sandstone. Even- 

 tually drainage along a few main lines or channels would be 

 •determined, and creeks formed such as we now know. Their 

 channels would be continually widening and undercutting 

 the porcelain covering, which, breaking ofl' for want of support, 

 ever presents a new face to wear, until at length the whole would 

 be removed, or a few symmetrically shaped conical hills with 

 their porcelain cappings be left standing about the plain in all 

 directions. 



Is not this the condition of Central Australia to-day — the 

 flat-topped hills with porcelain caps giving way to coarser-grained 

 grits with only glazed surfaces, and intermixed with calcareous 

 and ferruginous matter, as we approach the head of the present 

 drainage system ? And are not the metamorphic rocks at this 

 source composed almost entirely of bisilicates, which waters 

 charged with alkaline bicarbonates will rapidly attack and waste, 

 thus furnishing the material for the plienomena stated ? By such 

 reasoning one is tempted to regard the red quartzite as no other 

 than the bottom of the sea which washed still older rocks forming 

 the heart of the plateau to the northward, and which plateau, by 

 the uplifting of its southern seaboard, has been extended down to 

 the Macdonnell Ranges. The appearance of dry land in Central 

 Australia during secondary times must have an interesting bear- 

 ing on other features of Australia too great to discuss now. One 

 point may, however, be referred to, and that is the elevation which 

 the south shore of Australia is still undergoing, as evidenced 

 by the clifts of the Great Bight and other plienomena visible 

 along our littoral. 



The description Avhich Maw and Washington has given of the 

 Atlas Mountains and plains of Morocco are wonderfully like the 

 regions dealt with in this paper. Chalcedony-capped flat-topped 

 hills dot the plain, and igneous rocks raise Cretaceous strata to 

 heights of 5,000 feet ; while French geologists have on the eastern 

 end seen Jurassic strata still higher, and pierced with diorite 

 rocks. 



One subject which is of great interest to geologists is the 

 occurrence of obsidian l)ombs scattered throughout the wliole 



