Forbes-^ Appearance of Gold in the Earth's Crust. 887 



have arisen and been rendered auriferous solely from their 

 proximity to invisible or now superficial granites.' 



(2). The newer, or Dioritic, outburst I have called Post-oolitic as 

 the veins containing gold, and which proceed from its centres, cut 

 through strata contauung fossils of decided Post-oolitic forms, and 

 possibly may be as late as early Cretaceous. These strata are fre- 

 quently much altered and metamorphosed by the contact of the 

 igneous Diorite, and, at such points, often become auriferous, or are 

 cut by auriferous veins proceeding from the Diorite head mass. 

 Although the results of an extended examination of these deposits in 

 Chile, Bolivia, and Peru, occupying me from 1857 to- 1863, are 

 extremely interesting, I have only had time to publish comparatively 

 few of the observations made. Since my return to Europe, however, 

 I have been able to collect sufficient data to show me that this oc- 

 currence of gold is not at all confined J ^ South America, as I had 

 at first imagined, but appears also to fee common to all the other 

 quarters of the world. I have seen arririferous Diorites from Italy, 

 and some auriferous rocks of this class are known to occur in the 

 Ural ; and, as before-mentioned, I tS^ve specimens from California, 

 and I some time back received v^ry similar specimens, through 

 Lieutenant Aytoun, from the gold districts of India ; and, lastly, 

 within a few days, I have had the opportunity of examining a 

 fine series sent over to the Jermyn Street Museum by Mr. Aveline, 

 the head of the Geological Survey in Victoria, which are all stri- 

 kingly similar to those examined by myself in various, parts of 

 South America. 



11, Tore. Place, Portman Square, London. 



II.^Besults of Observations on the Cliffs, Gorges, and 

 .Valleys of "Wales. 



By D. Mackintosh, F.G.S.. 



[PLATE XV.] 



THIS article will be devoted to the consideration of the indications 

 of marine and fluviatile denudation furnished by the cliffs, 

 gorges, valleys, and other phenomena, of some parts of Central and 

 North Wales. With the view of collecting facts, the author, during 

 April and May of the present year, resided successively at Builth, 

 Pthayader, Aberystwyth, Dolgelley, Newtown, and Llangollen, so as 

 to have opportunities for repeated observations ; and the following 

 notes refer chiefly ta the neighbourhood of these towns and the 

 intervening districts. 



Abereddio Cliffs. — On entering the narrow part of the valley of 

 the Wye beyond Three Cocks Junction, I began, to be struck with 

 the dim outlines of terraces at a considerable elevation above the 

 river, until the romantic tiers of cliffs near Abereddw suddenly came 

 into sight. They are the culminating pouit of what, in their absence, 



^ See Forbes on Peru and Bolivia, Quart, Journ. Geol. Soc, vol. xvii. 



