248 J. F. KEMP AFTER-EFFECTS OF IGNEOUS INTRUSION 



that most ore-deposits and 'mineral veins,' as a class, represent one or the 

 other of the extreme products of magmatic differentiation, and that the most 

 striking chemical differences between ore-deposits is due (in the more impor- 

 tant class representing the siliceous extreme) to successive precipitation in 

 theoretically vertical zones, as the fluid migrates toward the surface, and with 

 diminishing heat achieves more and more mature crystallization. Secondarily, 

 the differences between ore deposits in different regions depends upon the 

 different metallographic magma-provinces." 



For metallographic we now generally use metallogenetic. The author 

 establishes earlier in the paper six zones of contrasted ores., depending on 

 the remoteness of precipitation from the igneous source, a series which 

 in whole or in part has been corroborated in many localities and by not a 

 few observers, of whom I am one. The nearest to the igneous source are : 

 1, the pegmatites; 2, the free-gold auriferous pyrite zone with coarse 

 quartz; 3, the cupriferous pyrite zone; 4, the galena-blende zone, often 

 with silver ; 5, rich silver and gold veins, with compounds of arsenic, anti- 

 mony, bismuth, tellurium, and selenium, all "mobile" elements; 6, zone 

 of barren earthy gangue minerals. The appreciation of magmatic emis- 

 sions has cast no less light on the difficult problems of the mineral veins 

 than on the obscure phenomena of the ancient metamorphic strata. 



If you will recall my description above of the crystallizing of a magma 

 in which mineralizers were dissolved, you will have before you the proc- 

 esses of gradual eviction. We now see that in their escape they take with 

 them the minute amounts of the rarer metals dissolved in the mass and 

 great quantities of silica and iron. Under circumstances of high tem- 

 perature and high pressure, both favorable to effective solution, the min- 

 eralizers, or magmatic waters, may carry their dissolved burdens through 

 the outer cooled shell, already cracked by shrinkage, and on through 

 faults, bedding planes, and porous rocks of the inclosing walls. If in- 

 tractable sandstones are met, quartzites are the chief result. If limestones 

 or other limy sediments are encountered, contact zones, replacement ore- 

 bodies, and bed-veins may be formed. In faults, fissure veins may be 

 precipitated in any wall-rock. At the maximum of emission, vein forma- 

 tion goes relatively far. With waning heat, earlier, remoter zones may 

 be repeated nearer the source. 



WATER IN SOLID SOLUTION IN PITCH STON US 



Now, if an intrusive magma rich in dissolved water gas be suddenly 

 chilled, as it sometimes is in a dike, and so quickly that no crystallization 

 is set up, then we have pitchstones with from 5 to 10 per cent of water 

 held in solid solution in the resulting glass. From pitchstones we get our 



