Pake. — Contact Metamorphism. 19 



Among ore-deposits genetically connected with eruptive 

 after-actions Vogt :1: includes cassiterite and apatite veins and 

 " ore - deposits of contact-metamorphic zone." Cassiterite 

 deposits are everywhere connected with acid eruptives, princi- 

 pally granite, and occasionally quartz-porphyry and rhyolite. 

 Partly for this reason, and partly because of the characteristic 

 paragenesis of fluoride, borate, and phosphate minerals, he 

 supports the common view that tin-deposits are genetically 

 connected with granitic eruptions, and that various volatile 

 fluorides took part in their formation. Cassiterite veins were 

 formed, he thinks, by pneumatolytic processes! — that is, by 

 the action of gases and water at high temperature and pres- 

 sure. He further urges that they were formed immediately 

 after the eruption, and before the complete cooling of the 

 granite, one proof of which is the occurrence of tin-vein 

 minerals in veins of pegmatite in the granite. 



Cassiterite veins are admittedly independent of the imme- 

 diately adjacent country rock, and for this reason seem to be 

 more nearly related to deposits of magmatic segregation than 

 to contact-metamorphic deposits. 



It is probable that the magmatic segregation of chromite 

 in peridotite was in some cases effected by pneumatolytic 

 agencies before the complete cooling of the magma. It is not 

 uncommon to find chromite in vein-like masses that have the 

 appearance of having been segregated in cavities of contrac- 

 tion in the pasty magma. As the agency of underground 

 water cannot have been active in this class of ore-deposit, the 

 aggregation must have been effected by metal-bearing steam 

 and gases occluded in the igneous magma. 



Pegmatite veins, while genetically connected with granitic 

 eruptions, seem to be of later formation than the cassiterite 

 veins. They often pass into quartz, and frequently possess 

 sharp well-defined walls, which suggest their formation in 

 shrinkage-cracks by pneumato-hydatogenetic agencies in the 

 waning phases of the after-actions developed by the pro- 

 gressive cooling of the eruptive magma. The different phases 

 of after-action must necessarily merge into each other, and 

 hence we may expect to find, as we do, tin- vein minerals and 

 even cassiterite in veins of pegmatite. 



Among ore-deposits of contact-metamorphic origin Vogt 

 includes the ore-bodies which occur within the metamor- 

 phosed contact zone of deep eruptives, especially granite. He 

 distinguishes several types of contact deposit'. The Chris- 



* J. H. L. Vogt, "The Genesis of Ore-deposits," New York, 1901, 

 p. 636. 



t " Pneumatolysis " is a term first used by Bunsen to describe the 

 combined action of gases and water. 



