170 HORN EXPEDITION — SUMMARY. 



{jliice (luriii;^ tlic later stages i)f ck'j>ijsiti(nis uf the Desert .Sandstijiie wliilst Messrs. 

 Tate iiiitl Watt ha\c argued that a considerable interval must have elapsed between 

 the formation of tlie Desert hsandstone and the silieification ; (2) Messrs. Tate ajid 

 Watt point out that the formation of agates and obsidian bombs ami of the 

 Desert Sandstone Breccia recjuire a eonnnon origin and suggest that though there 

 are gi-('a,t ditiiculties in the way of its acceptance because of the " widespread 

 silieification and the actual absence ovei- its area of any traces of ;ictnal volcanic 

 outlniists " it is essential to assume the former existence of volcanic action, the 

 silicates of the ash beds or larva being the source of the re(juisite siliceous material. 

 "The obsidian bombs demand volcanic action .... The development of 

 agates within the volcanic material was only anotluu- {jhaseof siliceous precipitation. 

 Oi this suppositi(jus volcanic formation all th.i.t rc^mains are the agates and the 

 obsidian bomlw." 



Tertiary. — Professor Tate has on previous occasions drawn attention to the fact 

 that in what he terms th(^ newer Pliocene times pknial conditions prevailed over 

 the central area. Indications of this are to be seen in the form of gravels through 

 wliich the present river channels have cut their way, and in the form of terraces 

 along the margin of the broad valleys along which now wander the reduced water 

 courses. At that time Lake Eyre must have been an inland sea, and fcssil 

 remains i>rove that it was inhabited by alligators now extinct iyPalliiiDiarchus 

 pollens), and by such tish as Ceratodus, while the land was inhabited by a marsu- 

 pial fauna consisting of gt'iicra such fis Diprotodon, the larger number of which 

 are now extinct. The river (Joyder close to where we crossed it ran between clills 

 about thirty feet high composed of river detritus ; on the north side of the escarp- 

 ment at Crown Point, a well-dekned shingle beach rises to an elevation of fifty 

 feet, while three miles south of this the Yellow Clitl', fifty feet high, bounding the 

 southern liank of the water-course, consists of tumultuously bedded sandstones and 

 conglomei'ate. 



The former existence of a considerable lacustrine area is shov/n also Ijy a 

 fossil deposit at Dalhousie, which has the nature of a gypsiferous tull' containing 

 numerous shells of iMilaind veni/sliila, M. lidi'Sd, M. Inildiiiu'iisis, Bithinia aiislralis 

 and Corbiiula mbhcvii^ata. None of these were found in the waters of the mound 

 spring close to which is the deposit, nor are they found living in the iuuuediate 

 neighbourhootl, while AI. liilosa does not now occur in the central area. 



Palaontology. — In I'egard to this the more important facts as detailed by 

 Professor Tate are as follows. 



