58 HORN EXPEDITION — GENERAL GEOLOGY. 



adjacent sandstones are distinctly traceable, the dip not exceeding 10°. These 

 structures must liave resulted from silicification passing downwards along vertical 

 planes of limited width, so that alternations of quartzite and unaltered sand- 

 stone occui', and that subsequent denudation has removed the sofi^r intervening 

 material. 



3. Red-bank Gorge Section. — A section of the strata exposed in the gorge, 

 through which the Red-bank Creek finds its way through the range, was examined, 

 together with the exposures to the south. The strata here are much disturbed and 

 altered, and the bedding-planes of the quartzite, in which for the most part the 

 gorge has been cut, can only be made out with great difficulty. Further south the 

 quartzite gives place to micaceous clay-slate, the junction line between the two 

 rocks revealing the amount and direction of dip. Bands of much-crushed quartzite 

 and clay-slate, dipping at an angle of 32° N.N.E., alternate with one another as 

 we travel southwards. At the southern extremity of the gorge the quartzite is 

 entirely replaced by micaceous clay-slate, wliich is in places highly charged with 

 oxide of iron, and some part of which is no doubt identical with the ironstone hill 

 described by Mr. Tietkens (xii., p. 10). Underlying this clay-slate are thick beds 

 of much-disturbed and contorted raagnesian limestone containing large concre- 

 tionary masses of oxide of iron. White masses of magnesite, wliich have weathered 

 out of the limestone, occur on the surface. Interbedded with this limestone are 

 thin bands of a strongly-laminated yellow micaceous shale. The limestone has 

 been much cracked, and into the usually narrow crevices thus produced anhydrous 

 and hydrous varieties of silica have filtered, forming numerous veins, which inter- 

 sect the limestone in an irregular manner. Further soutli the surface of tlie 

 country is covered by sand, gravel, and river-alluvium of the Davenport and its 

 numerous tributaries. 



In the valley of the Davenport, west of Mount Sonder, gneissic granite was 

 observed outcropping at the surface, being in all probability an inlier of the Pre- 

 Cambrian group. In the range through which the Finke River flows (when there 

 is any running water in its channel) quartzite occurs shortly below the junction of 

 the Ormiston and the Davenport, dipping at very high angles to the north. 

 Further south along the Finke Gorge fossiliferous quartzite succeeds, dipping 70° 

 N. 20° E., this being underlain by thinly-bedded and ripple-marked quartzite, and 

 by quartzite with annelide burrows. Below this latter we find silicio-argillaceous 

 limestone passing into quartzite, and further soutli still there are grey quartzites 

 dipping 66° N. 20° E. 



