1G4 HORN EXPEDITION — SUMMARY. 



centre to the soutli of the Pic G.unlniiui area. " Bcgimiini;- from the iiortli these 

 comprise tlie qiiartzite ridge wliich forms the soutlierii hoiiii(huy (if tlie Prc- 

 Camlirian ai'ca ami in whicli are the i^Ieavitree, Emily, Tciinile liar, etc., gaps. 

 This ridge is succeeded on tlie south liy tlie Waterliouse, James, (Jeorge CJill, Le\i 



and Chandler Ranges They have a mean comliined width, if we 



include the iiiteivening plains and A'allcys of from si.xty to seventy miles. The 

 area occupied by tliem, therefin'c, must )>o more than 15,000 sipiare miles."* 



tycrtain of these strata have fieen previously a,ssigned hy Messrs. H. Y. L. 

 Brown and Chewings to the Oamhriau period but the subsequent discovery of 

 fossils of Ordovician age in certain of these rocks and of waterworn fragments of 

 Ordovician limestone containing characteristic fossils in others show that this 

 determination was erroneous. 



In 1891 Professor Tate referied certain fossils obtained by INIr. Clunvings at 

 tlie head of the Walker Rivei, Mereenie Blufi' and Petermaiin Cre(di to the Upper 

 Silurian, but in the same year Mr. 11. Etheridge, -lunr., referi-ed fossils secured by 

 Mr. II. Y. L. Brown from the same horizon to lh(^ Lower Sili/riau age, ;ind the 

 latter author then ref(>rred the rocks of the (Jeorge (Jill, the James and the 

 Ooraminna Ranges to the same age. 



This determination of the Lower Silurian age of the fossil bearing rocks has 

 been confirmed and adopted in the report. 



Messrs. Tate and Watt now eliminate Cnmbrian from the classification of the 

 rocks and " include in the Drdovician system all the strata lying between Mount 

 Burrell cattle station on the soutli and the McDonnell Banges on the north, with 

 the exception of the conglomerate which was observed on the noi'th side of 

 Rudall Creek and on the banks of Ellery Creek north of the Luther.an Mission 

 Station (Ilerinannsburg)." 



The Ordo\ician rocks consist for the most part of quartzites and sandstones 

 with beds of limestone, clay-slate, micaceous slates and .sandstone. Thus for 

 example in the section ((Jeology, Plate L, Fig. T)) across the McDonnell Range in 

 the neighbourhood of Mount .Sonder and scmth to the Missionary Plains, the Pre- 

 Cand)rian gneiss and mica-schist are seen lying to the north of the range. The 

 high ridge is formed mainly of Ordovician quai'tzite replaced on the south l)y 

 micaceous clay-slate, underlying which are thick beds of magiu'sian limestone 

 which pass to the south under the river alluvium, forming the valley along \\hicli 

 flows the Davenport Creek. Gnei.ssic granite outcrops in this valley repi'esenting 



* I'art in., l'li,\sical (aom-.-i|iliy, pane fj. 



