no 



As will be seen the greatest difference consists in the 

 amount of water which is much smaller in the gyrolite from 

 Niakornat than in the others; accordingly the amount of the 

 other constituents must, in general, be somewhat larger; an- 

 other difference, which however cannot be considered as very 

 essential, is that comparatively much of the CaO is replaced 

 by ^1/2^3 and Na^O. The formulæ of the minerals can, quite 

 exactly, be expressed as follows: 



Gyrolite from Niakornat Z SiO^_, 2CaO, 1Ч2Е2О, 



Gyrolite from Skye, N. Scotia, N. Al maden 



ЗАгО,, 2 CaO, 2^/2— ЗЯ^ОМ, 

 Gyrolite from Brazil and Fort Point 



ZSiO.,, 2 CaO, 24éH.,0. 



As long as we have not found transitions between the 

 different forms it is not possible exactly to decide if they are 

 in reality the same mineral; 1 find it very probable that the 

 gyrolite from Niakornat represents the fresh state of the 

 mineral and that all the other gyrolites are altered by assorption 

 of water, by that alteration the specific gravity must, of course, 

 be lowered; the crystalline structure has been so much de- 

 formed that the etching and percussion figures cannot be made 

 in the altered mineral, while the optical properties seem to 

 be almost the same in both varieties, all forms of gyrolite 

 being optically negative and either perfectly uniaxial or occa- 

 sionally weakly biaxial. It is true that no similar alteration in 

 other minerals seems to be known; but the great similarity 

 between the different varieties of gyrolite in respect of occur- 

 rence and crystalline form seems to show that they belong to 

 the same mineral species. 



^) hi the case of the mineral from Skye, Andersen observed aheady in 

 1857 that onethird of the water goes oil' at 100°. The formula there- 

 fore for the Skye mineral — and presumably for other gu'olites with 

 the same proportion of H.^O — should empirically be written H^Ca2 



