T. Mellard Reade — Oceanic ' Deeps' 19 



channel by whicli they could have reached that position by the aid of 

 circulating water. The freshest norite of the region is found close to 

 ore bodies and enclosing parts of the ore. Even so susceptible 

 a mineral as hypersthene stands unchanged beside inclusions of 

 pyrrhotite and chalcopyrite. In thin sections of the freshest specimens 

 the sulphides are found embedded in original rock-forming minerals, 

 such as hypersthene, augite, biotite, and even titaniferous magnetite, 

 with no decomposition products between. 



It is noteworthy that the ores, the heaviest parts of the original 

 magma, everywhere occupy the lowest points in the eruptive sheet, 

 bays projecting into the country rock, or long and sometimes inter- 

 rupted offsets from the basic edge of the sheet, showing that gravity 

 was an important segregative force. Even in the narrowest offsets, 

 however, there is always some norite to show the connection with the 

 main body of that rock. 



Ore bodies never occur at a distance from the norite-micropegmatite 

 sheet, and not one has been discovered elsewhere in northern Ontario 

 after long and careful prospecting. That this eruptive mass was the 

 source of the ore is evident, and it is equally evident that ore and rock 

 reached their present position in a molten condition. It should be 

 added, however, that a certain amount of later rearrangement of the 

 sulphides has taken place, though there has not been sufficient water 

 action to cause any banding or ' crustification ' nor to introduce any 

 appreciable amount of gangue minerals, such as quartz or calcite. 



Practically every geologist who has visited the Sudbury ore deposits 

 agrees with Dr. Barlow and myself as to their magmatic origin, and 

 the only objections made to the theory have come from those who have 

 studied specimens apart from their field relationships, or who have 

 drawn inferences from the present arrangement of the ores as shown 

 on polished surfaces. To determine the present arrangement of the 

 ores throws very little light on their origin, for we are all agreed that 

 a certain amount of solution and redeposition has gone on, especially 

 in offset deposits, like the Copper Cliff mine. Any theory of original 

 deposition of the ores from circulating waters must give a reason 

 for the constant association of the ore with a single sheet of eruptive 

 rock, for its presence only at the lowest points on its edge, for its 

 blending upwards into the eruptive, and for the isolated blebs and 

 masses enclosed in the fresh eruptive. All these relationships are 

 easily explained by the magmatic theory, but very difficult to account 

 for by any other. 



IV. — Oceanic 'Deeps.' 

 By T. Mellard Reade, F.G.S., F.R.I.B.A., A.M.I.C.E. 



ONE of the most characteristic features of the great oceans is the 

 presence of what have been aptly termed ' deeps,' enormous 

 depressions in the ocean bottoms. Having in my " Evolution of Earth 

 Structure " ^ discussed their distribution and probable origin, I was 



1 Longmans, 1903. 



