822 Geological Society : — ■ 



insignificant ore-occurrences, including the stanniferous magnetite in 

 the older granite -gneiss of Ross-shire) date from the pre-Cambrian. 

 A group of pyritic fahlblands in the Highlands of Scotland, and the 

 wolfram-bearing pegmatites of the Grainsgill greisen, date from 

 the Caledonian, accompanying the widespread province of Caledonian 

 granites. The great bulk of the deposits of economic importance, 

 including the veins of Cornwall and Devon, the lead, zinc, and 

 copper veins in England, Southern Scotland, Wales, and Ireland, 

 are of Hercynian (and Armorican) age. This is shown by the 

 age of the fissuring in many cases (post-Carboniferous to pre- 

 Triassic), by the absence of ore-veins in Jurassic or later formations, 

 and by other evidence. The Tertiary volcanic period was not accom- 

 panied by ore-deposition, the evidence which has been adduced in 

 favour of this in the Isle of Man, Cornwall and Devon, and the 

 North of England, being unsatisfactory. 



The ore-deposits are classified in accordance with the above- 

 mentioned metallogenetie epochs, and are divided into metallogenetic 

 provinces, as has been done by Prof. L. de Launay with the ore- 

 deposits of Italy, Africa, and Siberia. The essential features of the 

 different groups are summed up. The evidence, collected and sifted, 

 indicates the following zones of ore-deposition :- — 



(1) Pneumatolytie zone : tin, passing up into copper. 



(2) Deeper vein-zone : copper with gold. Lead and zinc subordinate. 



(3) Middle and upper vein-zones : lead and zinc. Copper subordinate. 



The conclusions drawn from the investigation are : — 

 (i.) The importance of the physical conditions of the Permo-Trias 

 in favouring ore-deposition in upper zones. 



(ii.) The close connexion between metallogenetic and petro- 

 graphical provinces, and the essential dependence of ore-formation 

 throughout geological time on the differentiation of igneous rocks 

 accompanying great crust al movements. Differences in ore-deposits 

 in different localities and regions appear to be due to primary 

 differentiation of ores accompanying the differentiation of igneous 

 magmas at successive epochs. 



3. ' The Geological Structure of Southern Rhodesia.' By 

 Frederic Philip Menuell, E.G.S. 



The author describes in some detail a portion of what may be 

 termed ' the Laurentian Area' of Africa. The oldest rocks include 

 all lithological varieties, and exhibit most of the known types of 

 alteration. They comprise a great development of hornhlendic 

 rocks (epidiorites and amphibolitcs) ; on the other hand mica- 

 schist^, and sheared rocks generally, are conspicuously absent. They 

 include (1) ' basement schists ' on which the altered sediments were 

 laid down, and (2) altered basic igneous intrusions, simulating 

 rocks of any previous age. All these are older than the granites 

 by which they, and the metamorphic series, are invaded. 



The vertically bedded ' ironstone series ' is described, and is com- 

 pared with similar rocks of the Lake Superior region. They are 



