24 ROYAL SOCIETY OF CANADA 



of the colour material, shading oil' from deep azAire blue through pale blue 

 to colourless. Occasionally crystals exhibit a very decided and beau- 

 tiful zonal arrangement. The hardness of the mineral is s = 9. The 

 specific gravity of the blue corundum from the nepheline syenite of 

 Dungannon ranges from 3.93 to 4.01, with an average of 3.95. 



A microscopical examination of the thin sections of the rock shows 

 that in addition to the larger and more perfect crystals which are visible 

 to the naked eye, there are often innumerable small, usually exceedingly 

 irregular individuals distributed through the rock. 



Corundum under the microscope has a high index of refraction 

 but a low double refraction, and in good thin sections the interference 

 colours do not exceed a red of the first order. Such sections are, how- 

 ever, difficult to obtain on account of the relatively much greater hard- 

 ness of corundum as compared with the surrounding minerals. When 

 these latter^ have been ground sufficiently thin the corundum grains, as 

 may be seen in sections from which the cover glass has been 

 removed, stand out in relief, the result of their resistance 

 to the grinding operations. As a consequence, therefore, 

 the corundum seems to have a higher double refraction than it actually 

 possesses, and the mineral in most thin sections shows very brilliant 

 polarization colours between crossed niçois. The pronounced relief, 

 the dark borders of total reflection, the rough surface and the parting 

 planes or psou do-cleavages are very strongly marked, as is also the nega- 

 tive character of the double refraction. The following localities show 

 corundum in the nepheline syenite, most of which, are of economic im- 

 portance: — Lot 12, cons. XI and XII; lot 18, con. XI; lot 12, con. XIV 

 and XV; lots 6 and 7, con. XV; lots 6 and 7, con. XVI, Dungannon 

 township. Lot 4, con. I, Monteagle; lots 2 and 3, con, II, Monteagle. 

 Lot 34, con, V., lot 25, con. VI, and lot 32, con. VII of BnidencU town- 

 ship. (See Plates 10 and 12.) 



An analysis of the corundum from lot ]2, con. XV, of Dungannon 

 is given on page 68, and an analysis of the corundum from Craiginont, 

 in the township of Eaglan, on page 72. 



Calcitc — This mineral is very frequently present and is especially 

 abundant in those exposures which are in immediate contact with crys- 

 talline limestone. Its unexpected presence in the rock has already been 

 explained elsewhere (see page 12). That the mineral is foreign to the 

 magma and represents included fragments of the neighbouring crystalline 

 limestones is in direct agreement with all the phenomena so far observed. 

 Its mode of occurrence is essentially that of an original constituent, be- 

 ing found in comparatively large, well defined, unusually rounded grains, 

 sometimes coin])l('t('lv enclosed by the, other constituents, er in other 

 cases lying between ilieni. The line of separation is (|uile sharp and 



