IIOHN EXPEDITION — PETKOLUGV. 83 



the tonn " uraliic " can be justly used for such material. Typical uralite, of which 

 Nos. 73, lOo, 123, afford examples, is not strongly coloured, and usually retains 

 the form of the oi-iginal augite grains ; the planes of symmetry of the hornhlcndes 

 and augites being jiarallel. In the [irestiit cases, on the contrary, the hornblendes 

 do not appear to have any definite morphological relation to the augites, and they 

 are strongly coloured, and probahly contain much more iron than the latter. 

 These points would seem to indicate a re-crystallisation of the augite material, 

 together with either a bleaching of the residual augite, or extraneous addition of 

 iron, or both, rather than the gradual paraniorphic change of material from one 

 crystalline condition to another. Whether the residting hornblende, in one case 

 or the other, is filirous or not is probaljly unessential. 



In calling attention to this point we desire merely to jioint out th;it wc have 

 here rocks containing a large proportion of compact, strongly-coloured hornblende, 

 and that there is strong evidence that nnicli of it has been formed of material 

 previously crystallised in the form of augite. 



< )ur best thanks are due to Profes.sor T. W. E. David for his kindness in 

 allowing the sections to lie cut in the (ieological Laboratory of the Sydney 

 University, and to Professor Judd, who so kindly placed at our disposal micro- 

 scopes and a room at South Kensington for the use of the artist engaged in 

 lithogra[)hing the plates. 



SEDIMENTARY ROCKS. 



Arkose. No. 213. Plate IT[., Fig. 3. 8p. gr. 2 Gl. 



Ayers Rock, a physical feature of great interest, a tlescription of w hich is 

 given in the chapter dealing with the "Physical Geography,' is entirely made up 

 of this rock. A short note is given on this rock because it has be(Mi so oftt'ii 

 called a granite, and, though bearing a somewhat superficial resemblance to a 

 granite in its macroscopical features, on examination is found to be a i-ock of 

 sedimentary oiigin. It is composed of more or less rounded grains of quart/, and 

 felspar, with a large amount of cementing material, consisting of brown and red 

 oxides of iron and a little magnetite, probably derived from l)iotite which has 

 entirely dis;ippeared. The felspar grains consist of microcline, some of them 

 remaining fairly fresh, whilst others are considerably kaolinised. Some of the 

 microcline grains show the carlsbad twinijig in addition to the characteristic 

 micro-structure. 



