HORN EXPEDITION — GEXERAL riEOLOriV, 71 



Tlie devflopiiiPiit of agates witliiii tlio volcanic material was only another phaw(^ nf 

 siliceous pi'ccipitation. Of this suppositious volcanic formation all that remains 

 ai'e the agates and the obsidian bombs. The theory seems wild in the extreme 

 because of the wide-spread silicification, and tlie absence over its area of any traces 

 of actual volcanic outbursts ; nevertheless, no other explanation accounting for the 

 several phenomena appeal's to us admissilile. 



YITT.— Tertiary. 



Excepting tlie silt deposits of the present water-ways and the widespread 

 sand-plains, the only Tertiary deposits of any significance are those which indicate 

 a former water-flow of vaster volumes than at present. These signs are chielly in 

 the form of gravels, more or less consolidated, through which the present water- 

 channels have cut tlieir way, or in the form of terraces margining the valley-plains 

 through which now flow relatively diminutive ci-eeks. These facts demonstrate 

 that high pluvial conditions once prevailed ; and, in conseciuence, perennial flcnvs 

 in the river channels of this region were maintained, which, discharging into Lak(> 

 Eyre, and supplemented by an Artesian supply in and around it, jiroduced an 

 inland .sea of fresh water, inhabited by alligators ( rallimiiaichus pollens) and 

 turtles, and on its marshy margin dwelt Diprotodoii and its fossil associates. 

 Tnferentially the date of formation of these gravels and river-terraces is coeval 

 with the existence of Diprotodo)!, whose extinction was due to those physical 

 causes which destroyed its habitats, and gave Central Australia its present i-igor- 

 ously dry climate. The marsupial life of this period, on comparison with that 

 which replaced it, indicates a high anti(|uity in the number of extinct gcMiera, and 

 the very high percentage of extinct species, .so much so, that the oidy a])plical)le 

 time-term is (Newer) Pliocene, and not Pleistocene or Post- Tertiary as employed 

 by some writers. 



RiVEK Gravels. ^The River Coyder, where examined liy us, flows between 

 low cliff's of river detritus, and low mounds extending far b.ick from the existing 

 channel, are composed of the same material. A section at about (wo miles east 

 from the telegraph crossing comprises from al»ove dosvnwaiils :- -S.-mdstones some- 

 what evenly bedded, 5 feet ; conglomerate of subangular pebbles of (juartzitt^ 

 and Desert Sandstone, 1 foot ; fiialile white sandstone, current-bedded, W\\\\ a 

 coarse shai-p grit in the medi.al p.irt, "JO feet ; total, "20 f(»et. 



The south bank of tin' Finkc ]li\i'r, at ili-ubuiy, is coinposeil of gravel 

 ■2U feet high. 



