Guide books of excursions in Canada1-10 . -shaped hexagonalcrystals, several inches in length, may be seen on theseweathered surfaces. In the freshly broken rock, on the3ther hand, the corundum is detected only with difficulty,unless it assumes the usual bluish colour which enablesit to be readily distinguished from the other constituentminerals of the rock. In the exposures these rocks are seen to be cut throughin various directions by dykes of fresh red pegmatitecomposed chiefly of red feldspar (orthoclase), microperthiteand quartz, with a little hornblende. In some placesimperfect crystals
Guide books of excursions in Canada1-10 . -shaped hexagonalcrystals, several inches in length, may be seen on theseweathered surfaces. In the freshly broken rock, on the3ther hand, the corundum is detected only with difficulty,unless it assumes the usual bluish colour which enablesit to be readily distinguished from the other constituentminerals of the rock. In the exposures these rocks are seen to be cut throughin various directions by dykes of fresh red pegmatitecomposed chiefly of red feldspar (orthoclase), microperthiteand quartz, with a little hornblende. In some placesimperfect crystals of this last-named mineral can be foundwhich measure from 4 to 6 inches (10 to 15 cm.) in dykes are evidently differentiation forms of a redsyenite, which occurs in considerable volume immediatelyto the south of these exposures, and which is probably asomewhat quartzose type of the normal red variety ofalkali syenite. An analysis of this rock was made by Prof. Norton-Evans, with the following results, under I.— 35064—6. 83 99-66 II. 58-3223-801 III. Molecular Ratios •972 •233•007•023•119•029•015• 094 •972•240 ?280 Deducting the excess of alumina present as corundum,which was determined by trial, neglecting the loss onignition(H20) and deducting the amount of lime (CaO)necessary to form calcite with the CO2 in the rock, theresults given under II are obtained. This is the compo-sition of the residual or alumina-saturated magma. Morozewicz [9, p. I and 105] has shown by directexperiment that in super-saturated alumino-silicate mag-mas, whose general composition is RO, n AI2O3, Si02 (whereR = K2, Na2 or Ca; and n = 2), the whole of the excessof alumina separates out (1) as corundum if no consider-able amount of MgO or FeO is present and if n is lessthan 6; (2) as sillimanite and corundum if n is greaterthan 6; (3) when the magma is rich in magnesia, as spinelor spinel an
Size: 1183px × 2112px
Photo credit: © The Reading Room / Alamy / Afripics
License: Licensed
Model Released: No
Keywords: ., bookcentury1900, bookdecade1910, booksubjectgeology, bookyear1913
