nORX KXPEDITIOX — GKNEHAL fiKOLOOY. 53 



At (lill's Pass the DrdoNiriaii liinestoiios, dipping soutli, conic into contact 

 Ijy a fault with a glassy (juartzitc (forming a high, scarped ridge), which introduces 

 the Pi-c-(Jandji-ian roiks, which continue uninterruptedly northward to Burt Plain 

 with high planes of foliation dipping north (see Section from Missionary Plain to 

 Gill's Pass). Although the actual junction line was not obsiu-ved, there is strong 

 evidence that tlu' ()rdovici;in limestone and quiirtzitc and the Post-()rdo\ician 

 conglomerate, already referred to, are unconformaljle to each other. In the lowest 

 strata of this conglomerate among peljbles of other rocks were found waterworu 

 fossilifcrous fragments of the red crystalline limestone belonging to the Ordosiciaii 

 System. Thus the Ordovician limestone must have been consolidated, upheaved 

 above sea-level, and eroded before the formation of this conglomerate. 



The actual junction-line between the Ordovician and the Cretaceous was not 

 obser\'ed. The great difference in the amount of dip of the strata of the two 

 systems, even close to the junction, as observed a few miles south of Htudiuiy 

 Cattle Station on the Finke, gives some idea of the sti'ong unconformability that 

 must exist between the rocks of the two systems. At Henbury itself small outliers 

 of lioriz(^ntally bedded chaleedonized sandstone, so characteristic of the Ci'etaceous, 

 rest on Ordovician argillaceous limestone, dipping at a moderately high angle to 

 the south-east, the actual contact being concealed l)y the waste from the ovei'lying 

 bed. 



(</) Structural Features. 



Foldiui^. --The Oi'dovician rocks have been thrown into a number of 

 folds, gentle in the south as in the (ieoi'ge (lill antl Levi Ranges, and 

 bectjming sharper in the noi'th. In the- ranges just mentioned the rocks, 

 chiefly sandstone, dip at low angles, \iz., from 8' to 10° to the south on the north 

 sid(>, and about the same amount in an opposite direction on the south side. Thus 

 the rocks of these ranges form a shallow synclinal trough ; the corresponding arch, 

 probably originally situated on the north sid(^ of these ranges, has been entirely 

 removed by denudation and erosion, its place being now occupied by the valley of 

 Petermann Creek. At Mount iSonder and in the valley of the Davenport to the 

 south, at the opposite edge of the Oi'dovician area, the strata have sull'ered the 

 greatest amount of disturbance. There ([uai'tzite, alternating with micaccsous shale 

 and passing below into dolomitic Umestone, is dipping at high angles to the north 

 (see Section INlcDiinnell Range to Levi Range). 



Absence cf Eruptive Kocks. — A somewhat rcmarkabh^ feature of this ancient 

 system of rocks is the total aljsence, as far as observation goes, of erujitiNC rocks, 



