124 ME. J. EOMANES Olii THE [Feb. I9I2, 



These vary from rocks presenting the appearance of a fine purple 

 mudstone to a fairly coarse ashy sandstone, and were recorded by 

 the late Mr. Attwood, and petrographically described by the late 

 Mr, Hudleston^ as consisting largely of kaolinized soda-felspar. 

 The finer-grained types have a striking resemblance to some of the 

 deposits at El Brazil. The strata forming the Carballo are striking 

 north and south, and have a gentle dip of 10° eastwards. The 

 same series is well exposed at the mouth of the Eio Earranca, on 

 the south side in natural cliffs and on the north side in a new 

 railway-cutting. The strata here have the same dip and strike as 

 at the Carballo, and within a very small radius show an extra- 

 ordinary variety in appearance. On both banks of the river the 

 rock is of a brilliant yellow-white speckled colour, often mottled 

 with vivid red owing to iron-staining. When these beds are 

 followed south-eastwards along the shore, it is found that in 

 50 j^ards they have changed to a friable fossiliferous ash, and in 

 another 100 yards to a very compact, greenish, shelly deposit, all 

 this taking place without the least break of any sort. The fauna 

 of these beds shows the same peculiarity, the dominant forms 

 changing rapidly from place to place : all of which points strongly 

 to these beds being of an extremely shallow-water character. 



On the road from Barranca Bridge to the camp at Barranca- 

 Tunnel are several exposures of these sediments rising from the 

 alluvial flats of the river. These show, again, considerable diversity 

 of lithological character along this road. The dip and strike vary 

 greatly from place to place. In the first exposure the strike is 

 west 20° north, dip 30° southwards ; about a quarter of a mile 

 farther on the strike is east and west, with a dip of 20° southwards. 

 In this latter exposure the upper bed is a coarse brown ash, showing 

 remarkably clear spheroidal weathering ; below this comes a thin 

 intercalation of shale overlying a coarse greenish ash. At the 

 mouth of the tunnel the strata are striking north 50° east and 

 dipping 80° northwards. JN'owhere is any violent folding to be seen, 

 and so probably the beds are much faulted, giving rise to block 

 structure on a small scale. 



Petrology of the Barranca sediments. — All these different 

 types of deposits have one important feature in common. They 

 are all ashes, but with different texture and composition which 

 determine their appearance and special mode of weathering. 

 Specimens from the very pale beds at the mouth of the Barranca 

 appear in a slice to consist largely of fragments of chert, or at least 

 some form of cryptocrystalline silica, set in an earthy or ferruginous 

 BQatrix. Broken crystals of felspar and quartz are present, but are 

 by no means common in this type. Many of the siliceous fragments 

 contain irregular patches of calcite, and so they may perhaps 

 represent silicified limestone. These ashes contain some badly 

 preserved specimens of Glohirjerina. \ 



' In G. Attwood, Q. J. G. S. vol. xxxviii (1882) p. 339. 



