Geological Congress in Canada. 487 



Dr. Iddings in a paper oa "Some examples of Magmatic Differentiation 

 and their beai'ing on the problem of Petrographical Provinces " agreed 

 with Dr. H. S. Washington in emphasizing the importance of sufficient 

 analyses. In Dr. Washington's paper on "The Yolcanic Cycles in 

 Sardinia" he pointed out that there are three cycles — firstly, that 

 of the extensive early (Tertiary) flows; secondly, that of the two 

 large volcanoes of Monte Arci and Monte Ferru ; thirdly, that of the 

 small recent scoria cones. In the first two cycles the sequence began 

 with acid, followed by intermediate and basic rocks. The lavas of 

 the third cycle are felspar basalt in which no definite sequence has 

 as yet been made out. Rocks of typically Atlantic and others of 

 typically Pacific type occur in Sardinia, even in the same volcano, as 

 at Ferru and Arci. Professor W. H. Hobbs in a paper on "Variations 

 in Composition of Pelitic Sediments in relation to Magmatic 

 Differentiation " endeavoured to account for some of the variations in 

 igneous rocks usually attributed to differentiation by supposing many 

 igneous rocks to be the result of the fusion of argillaceous sediments. 

 Dr. y. Sabatini gave "A Classification of the Eruptive Rocks of Italy ". 

 The discussion was continued by Dr. J. W. Evans, Professor F. 

 Loewinson-Lessing, Professor A. Bergeat, Dr. W. Cross, and summed 

 up by Professor H. Backstrom, who advocated reserving judgment 

 until more experimental work has been done. 



The third subject discussed was the "Influence of Depth on the 

 Character of Metalliferous Deposits ". Professor J. F. Kemp opened 

 the discussion with a paper bearing that title. He concluded that 

 (1) while there seems to be nothing to prevent precipitation at greater 

 depths than we have yet reached, yet conditions seem to be specially 

 favourable in those portions which lie between the present surface 

 and 2,000-4,000 feet in depth ; (2) secondary enrichment has 

 increased the yield of those portions of many veins which are 

 above 1,000 feet in depth, the vertical extent of its action being 

 limited to a relatively short stretch below the ground-water level. 

 Professor J. P. Krusch followed with a paper on the colloidal 

 precipitation of primary and secondary ores. Professor W. H. 

 Emmons in " The Mineral Composition of Primary Ore as a factor 

 determining the Yertical Range of Metals deposited by Secondary 

 Processes " outlined the processes of enrichment of sulphide ores 

 of gold, silver, and copper, and reviewed some of the more important 

 experiments that may illustrate this process. Dr. L. L. Fermor 

 " On the Formation in Depth of Oxidized Ores and of Secondary 

 Limestones" stated and illustrated the thesis that when deposits 

 consisting of chemical sediments, such as oxides and carbonates as 

 of iron, manganese, or calcium, admixed with mechanical sediments, 

 such as sand and clay, are buried to a depth sufficient to bring them 

 into the zone of anamorphism, reactions take place, which frequently 

 necessitate the elimination from the oxides of oxygen in excess of 

 protoxide proportions, as from Fej O3 and Mn Og ; of carbon dioxide 

 from carbonates ; and of water from hydrated oxides, such as limonite. 

 It seems to be usually tacitly assumed that these escape or are 

 removed from the scene of action, but it is conceivable that the 

 pressure is such that they are unable to escape. When, in course 



