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ANNALS NEW YORK ACADEMY OF SCIENCES 



soon effects saturation, but the anorthite enters reactions by which new- 

 compounds are formed from it and its identity is destroyed. The original 

 crystals are thus placed in contact with a solution saturated with albite 

 but lacking anorthite, and with such a solution only pure albite is in 

 equilibrium. 



At the beginning of the recrystallization of the basalt induced by the 

 passage of heated aqueous solutions, the anorthite molecule appears to 

 have undergone reactions by which garnet and datolite and possibly other 

 compounds were produced from it. Albite under the new conditions was 

 stable. Anorthite was therefore continually leached out of the labra- 

 dorite, while albite was deposited in its place, a chief feature of the 

 process being probably a growtli of crystals at the expense of others in 

 the vicinity. 



Aaiu 



A/^fta 



Fig. 3. Basalt bordered by veiu of albite. prebiiite and natrolite. Albite is 

 developed from tbe labradorite of the basalt. Crystals of prebuite rest upon 

 it and botb are replaced by natrolite. X 35. Slide 127. 



In figs. 3-7 and in Plate XI, figs. 2, 3, albite is shown in characteristic 

 forms and in typical relations with the basalt and with secondary min- 

 erals. Slide 127, a portion of which is sketched in fig. 3, was taken from 

 a hand specimen in which the ordinary dense basalt is seen to be cut by 

 narrow veins of secondary minerals. Under the microscope, the basalt 

 presents the usual microcrystalline development, and the veins are found 

 to consist of natrolite, prebnite and plagioclase feldspar. The crystals of 

 the last occur in interlocking aggi'Cgates of varying crystallographic 

 orientation. They form borders to the basalt and project at various 

 angles into tbe veins, whose principal filling is natrolite. Crystals 0.5 

 mm. in length are not uncommon. Most of the crystals show albite twin- 



