54 HORN EXPEDITION— GENERAL GEOLOGY. 



whether plutoiiic or volcanic. There is no evidence either of the presence of 

 intrusive dykes or sills, or of interbedded lavas or tuffs throughout the whole area 

 examined. 



False-bedding. — False- or current-bedding is strongly developed in the sharp 

 gritty sandstones at Glen Edith, and on the north side of the range which forms 

 the southern boundary of Shake's Plain, which is known locally as the Station Range, 

 situated on the south side of Temjie Downs Cattle Station. There the current- 

 bedding is so well developed that one is apt at first sight to mistake the false- 

 bedding planes for the true-bedding planes. The former, however, are seen to 

 vary so rapidly in the direction of their dip that the mistake can be easily rectified, 

 and the true bedding planes made out without much trouble. 



Ripple-marks. — The surface markings known as ripple-marks are well 

 exhibited by the sandstone constituting the Levi and George Gill Ranges. In 

 th'i Levi Range, where they were particularly noted, the alternating ridges and 

 furrows were seen to have a very varying trend, at one time in a N.W. and S.E. 

 direction, and at another in a N.N.E. and S.S.W. direction. The width of the 

 ridges, as also of the furrows, was about one inch, and the height from the bottom 

 of the furrow to the crest of the ridge about half an inch. In George Gill Range, 

 where ripple-marks were seen to be extensively developed, the sandstone is evenly 

 and thinly bedded, and the surfaces of many succeeding laminae were seen to be 

 ripple-marked. The evidence deduced from the presence of these ripple-marks 

 indicates shallow water deposition of the strata in which they occur ; that is of 

 the sandstone of George Gill anil Levi Ranges. Ripple-marks were also observed 

 in the quartzite forming the walls of lleavitree Gap, and in the quartzite near the 

 northern extremity of the Finke Gorge. 



Concretionary Structure. — Concretionary masses composed chiefly of hydrated 

 oxide of iron occur in the fossiliferous liniestone on the south side of George Gill 

 Range. Concretions of a siliceous nature were observed in the siliceous limestone 

 Hanking the JVIount Gillen quartzite ridge on the south. A concretionary lime- 

 stone was also observed in the Finke Gorge. 



Sedimentation in Relation to Physiographic Changes. — A natural cycle of 

 sedimentation bearing a definite relation to the physiographic changes, through 

 which the locality under consideration has gone through, can be made out in 

 certain portions of the Ordovician strata. The northern half of George Gill 

 Range, for instance, is composed of sandstone dipping to the south. Tiiis is 

 followed on the north in regular descending order by calcai'eous shales or, perhaps 



