520 Reviews — The Placer Mines of Cariboo. 



Cambrian age. After three more Palaeozoic marine transgressions, 

 thrust-faulting occurred at the time of the Appalachian Revolution, 

 carrying Keweenawan beds over the Lower Palaeozoic. The present 

 land-surface is probably due for the most part to Cretaceous 

 denudation, which has exposed the outcrops of both principal faults 

 at different points. 



VI. — Notes on the Placer Mines of Cariboo, British Columbia. 

 By J. B. Tyrrell. Econ. Geol., vol. xiv, pp. 335-45, 1919. 



rjlHE importance of alluvial mining for gold in the valleys of the 

 JL Cariboo district of British Columbia has led to a careful 

 investigation and survey of the drainage system of this remote 

 region, revealing features of considerable interest to the student of 

 river development. An important feature of the region is the 

 existence of auriferous gravels of pre-Glacial age, in some instances 

 deposited by streams whose courses have been seriously modified at 

 a later date. Many of these gravels are now deeply buried under 

 boulder-clay and other glacial deposits. Some of the upper 

 tributaries of the Willow River in the neighbourhood of Mount Agnes 

 show very clear examples of capture and beheading, and in some cases 

 the present creeks have cut narrow gorges in the floors of wide open 

 valleys belonging to an earlier arrangement of the drainage. The 

 present stream system in the neighbourhood of Barkerville, as shown 

 in the small sketch-map given by Mr. Tyrrell, is a particularly 

 striking case of diversion. Although some of the smaller changes 

 seem to be due to the deposition of boulder-clay it would appear that 

 the larger modifications have been brought about more by piracy in 

 pre-Glacial times than by moraines or other barriers of Glacial age. 



VII. — Platinum Metals in the Somabula Diamondiferous Gravels- 

 By H. B. Maufe. Southern Rhodesia Geological Survey- 

 Short Report No. 5, 1319. 



THE Somabula Diamondiferous Gravels which contain the diamond 

 and numerous gemstones are found almost on the main water- 

 shed of Southern Rhodesia close to Willoughby's Siding and about 

 12 miles south-west of Gwelo. 



An examination of the pebbles composing the gravels revealed the 

 presence of chromite and chromite-bearing rocks in appreciable 

 quantity : and therefore the possibility that the rocks might contain 

 platinum or metals of the platinum group (iridium, osmium, 

 palladium, rhodium, and ruthenium). 



The late Mr. Zealley, in writing of the occurrence of platinum in 

 Southern Rhodesia, said: "The Somabula gravel for instance is a 

 likely source, since it is known that much heavy material is con- 

 centrated therein, and that a considerable proportion of the pebbles 

 are from ultra-basic rocks ; thus pebbles of chromite rock are 

 abundant, and many of the chalcedony pebbles can be recognized by 

 the practised eye as silicified serpentines derived from the Great 

 Dyke and from the ancient schists. The fine heavy black gold- 

 bearing sands concentrated from the Somabula gravels apparently 



