Revieius — Geology of the Counto^y West of Sinoia. 183 



The absence of proper petrographical examination detracts 

 somewhat from the value of the Bulletin as a contribution to science, 

 but no doubt it will prove of value to prospectors in South 

 Australia. 



Preliminary Report on the Geology of the Country West of 

 Sinoia, Lomagundi District. By A. J. C. Molyneux, F.G.S. 

 Southern Rhodesia Geological Survey, Bulletin No. 6. pp. 47, 

 with a folding sketch-map. Salisbury, 1919. 



rriHTS memoir contains a preliminary account of the geology of 

 -^ an area in Northern Mashonaland, including the greater 

 part of the " copper belt " of that region. It is interesting as 

 revealing the existence of some 34,000 feet of stratified rocks, 

 apparently unrepresented in Southern Rhodesia, but possibly 

 correlative with the Transvaal or Potchefstroom System of the 

 Union. The south-eastern portion of the area is occupied by 

 crystalline schists, mainly of igneous origin, but including a belt 

 of banded ironstone, probably sediments interbedded with the 

 lavas. The gold-bearing rock, commonly known as the Eldorado 

 conglomerate, is of uncertain origin, and the application of the 

 term " banket" to it is deprecated, as it has no features in common 

 with the Rand bankets. The overlying sedimentary rocks, which it 

 is proposed to call the Lomagundi System, comprise fragmental 

 and chemical deposits, including conglomerate, arkose, sandstone, 

 shale, slate, limestone, and the Sinoia Cave dolomite group. The 

 most important member is the uppermost or Piriwiri series, which 

 attains a thickness of over 20,000 feet. The Piriwiri mineral belt, 

 which is about 18 miles long and now worked to a considerable 

 extent, carries gold and copper, the most important occurrence 

 being the Eldorado mine, whose output to date has been about 

 432,000 ounces of gold. R. H. R. 



The Genesis of the Ores at Tonopah, Nevada. By E. S. 

 Bastin and F. B. Laney. U.S.G.S. Prof. Paper 104. pp. 50, 

 with xvi plates and 22 text-figures. Washington, 1918. 

 npHIS is an elaborate study illustrated by beautiful plates and 

 -*- figures of the genesis and relations of the ores of the well- 

 known silver-producing mining area of Tonopah, Nevada. The 

 author's conclusions, largely based on microscopic examination of 

 polished sections, may be summarized as follows : the great bulk 

 of the silver ores are of primary origin, but two types of primary 

 minerals are recognized. The alpha-hypogene group have remained 

 exactly as they were first deposited, including galena, blende, chalco- 

 pyrite, arsenopyrite, pyrargyrite, polybasite, argentite, wolframite, 

 and electrum, with, probably, stephanite and argyrodite. The beta- 

 hypogene minerals are those that have been formed by replacement 

 of the alpha group during the primary mineralization, and comprise 

 argentite, polybasite, chalcopyrite, and electrum. The minerals 



