16 Transactions. 



That sulphides can be segregated from eruptive magmas 

 in the first concentration has yet to be proved ;~ and it is still 

 doubtful how far Vogt's conclusions respecting the occurrence 

 of sulphide ore as products of primary segregation from 

 molten magmas are admissible. 



Art. III. — Contact Metamorphism in its Relation to the 

 Genesis of certain Ore-deposits. 



By Professor James Park, M.A.Inst. M.E., M.Inst.M. and 

 M. (Lond.), F.G.S., Director of the Otago University 

 School of Mines, New Zealand. 



[Read before the Otago Institute, 13th September, 1904.] 

 A molten magma tends to effect changes in the rocks with 

 which it comes in contact. In the case of overflow magmas 

 the thermal changes are generally trifling, and in many cases 

 hardly appreciable. Even magmas that have cooled in rents 

 in sedimentaries at shallow depths have not always caused 

 great changes in the enclosing rock. 



The greatest alteration will naturally take place in the case 

 of magmas that do not reach the surface, but cool slowly 

 under great pressure. The greater the mass of the intrusive 

 magma, the slower will be the rate of cooling ; and the 

 slower the rate of cooling, the longer will the adjacent 

 rocks be heated. The rate of cooling will be mainly de- 

 pendent upon the mass ol the intrusion, the distance from 

 the surface, and the relative thermal conductivity of the ad- 

 jacent rocks. 



The changes effected in the country rock by the intrusion 

 of an igneous magma will be mechanical and hydrothermal. 

 The intruded sedimentaries will be compressed, bent, and 

 more or less shattered and fissured along the line of in- 

 trusion. The magma will part with its heat by slow radiation 

 into the adjacent rocks. The magmatic steam and gases, to- 

 gether with the gases generated in the sedimentaries,'-- will 

 pass into and permeate the latter, and cause a molecular re- 

 arrangement of the constituent minerals, resulting in what is 

 termed contact metamorphism. As the igneous magma and 

 the heated sedimentaries cool they will contract in area, and 

 when the temperature normal to the depth has been reached 



* Professor Joseph Barrell has shown that the heat of an igneous 

 mass acting upon sedimentaries liberates enormous volumes of steam 

 and gases, attended by a shrinkage of volume of the rocks and the 

 formation of vein fissures : " The Physical Effects of Contact Meta- 

 morphism," Am. Jour. Sci., vol. xiii, April, 1902, p. 279. 



