M. H. Rastall — Differentiation and Ore-deposits. 297 



of crystals, but not the lower. Now there is evidence that during 

 their final crystallization some intrusions have become stratified 

 by gravitational sinking, as in the Palisade Traps of New York 

 and New Jersey, where the lower layers are very rich in olivine and 

 other heavy basic silicates, while the upper part of the sheet has 

 almost the composition of a granite, and yet there are no lines of 

 demarcation, but a gradual transition throughout. 



On the other hand, there is evidence that metallic compounds tend 

 to accumulate in the upper portions of granitic masses. In the great 

 granitic bathyliths of Western America it has been described how 

 the mineralization is concentrated in the upper portions of the 

 minor domes or cupolas that diversify the general surface of the 

 intrusion ; ^ this is obviously due to a gravitational rising of 

 the metalliferous fluids into the upper portions of the granite. It 

 is immaterial whether this occurred before or after the general 

 solidification of the rock. 



Hitherto we have spoken mainly of ore-deposits occurring in 

 definite dykes and veins or of segregations within or at the base of 

 igneous masses. There yet remain other important classes, such as 

 the brecciated lodes and the endless varieties of so-called contact 

 and replacement deposits. It is not proposed to discuss at length 

 the genesis of these. The lode-deposits of the type here indicated 

 can be explained on essentially similar principles: vapours or solutions 

 differentiated from igneous magmas penetrate into zones or belts of 

 fractured rock, of the type commonly now described as " shatter 

 belts ". Various more or less far-reaching chemical changes were 

 induced in the minerals of the country rock, while the constituents 

 of the solutions were deposited as ore and gangue minerals in the 

 interstitial spaces. Such lode-deposits often show no definite 

 boundaries, but shade off gradually into the unaltered country 

 rock. On the other hand, most of the replacement and contact 

 deposits come under the petrological category of metasomatism, 

 a special type of metamorphism involving a change in the total 

 bulk-composition of the rock. This is a large subject, needing 

 extended treatment, which cannot be followed further at this time. 

 The point to be emphasized, however, is this, that the fluids giving 

 rise to these types of lode-formation, contact, and replacement 

 deposits are of the same nature and origin as those that have filled 

 the sharply defined metalliferous veins, all being derived from igneous 

 magmas. Of course, it is not here maintained that all ore-deposits 

 are of direct igneous origin ; doubtless there are many exceptions, 

 but the evidence at present available goes to show that a vast number 

 are so. Also no account is here taken of the highly important 

 changes induced by oxidation, leaching, and secondary deposition. 

 Only primary unoxidized ores are specifically referred to. 



In the accompanying diagram an attempt is made to 



V Butler, Econ. OeoL, vol. x, 1915, pp. 101-22, 



