Penfield and Foote — Composition of Tourmaline. 121 



not more than traces of this element have been found ; these 

 may therefore be designated as Lithia Tourmalines, which 

 form a natural group. In tliis group B/ is higher than in 

 other varieties, while H is somewhat higher than the general 

 average ; W is very high and R" correspondingly low. This 

 variety of tourmaline has its particular mode of occurrence, 

 being found in pegmatite veins associated with quartz, albite, 

 microcline, orthoclase, muscovite and lepidolite. The material 

 is generally delicately colored and often transparent and of 

 gem-like quality. It is difficultly fusible when heated before 

 the blowpipe, fusibility =5-5^. 



At the opposite extreme, 31-34, are varieties which may be 

 designated as Magnesia Tourmalines. In these R", which is 

 chiefly magnesium, is very high and W correspondingly low, 

 while R' reaches its lowest limit, 0*2 to - 4. These varieties 

 are light-colored and at times of gem-like quality. They are 

 easily fusible before the blowpipe, fusibility = 3. With these 

 No. 35 should be associated, for it differs only in containing 

 considerable iron which is isomorphous with the magnesium, 

 hence the color of this tourmaline is black. The last live are 

 alike in their mode of occurrence. They have probably been 

 formed in limestones containing magnesium by the contact 

 action of intruded igneous masses during the pneumatolitic 

 period, when such masses were giving off heated aqueous 

 vapors containing boracic acid and fluorine compounds. They 

 are found in coarse crystalline limestone, associated with 

 graphite, phlogopite, pyroxene, amphibole, scapolite and apa- 

 tite. Contact metamorphisms of this nature have recently 

 been described by Lacroix.* 



The intermediate varieties (Nos. 14 to 30) with the excep- 

 tion of No. 21 are black or dark brown, owing to the presence 

 of iron. These are the ordinary tourmalines found in granites, 

 gneisses and schists, and sometimes in pegmatite veins inti- 

 mately associated with lithia tourmaline, as at Auburn and 

 Paris, Maine. They too have probably resulted from the min- 

 eralizing action of heated aqueous vapors containing boracic 

 acid and fluorine compounds, given off during the pneumato- 

 litic period of cooling igneous rocks. A contact metamorph- 

 ism of this character, attended by the formation of tourmaline 

 vein stone, has been very carefully described by Hawes.f 



Nos. 14 to 22 (No. 2L excepted) are characterized by con- 

 taining iron and only a little magnesia, hence they may be 

 designated as Iron Tourmalines / Nos. 23 to 30 contain both 



* Le Granites des Pyrenees et ses Phenomenes de Contact, Bull. Carte Geolo- 

 gique de France, No. 64, Tome x, p. 54, 1898. 



f The Albany Granite, New Hampshire, and its Contact Phenomena, this 

 Journal, xxi, p 21, 1881. 



