36 CLASSIFICATION' BASED OX MINERAL COMPOSITION. 



(interlaminations or iutercrystallizations) of orthoclase and albite. Alblte 

 and anorthite wei'e looked ujion as two distinct species of triclinic feldspar, 

 and it was held that labradorite, andesite, and oligoclase were formed from 

 isomorphous mixtures of albite and anorthite, — that is, through tlie mo- 

 lecular union of albite and anorthite, in definite mathematical pro^Dor- 

 tions. These mixtures Tschermak distinctly held were not mechanical, but 

 molecular. 



The finding of potash in the triclinic crystals formed from the molecular 

 union of albite and anorthite was explained by Tschermak, by the supposi- 

 tion that some orthoclase was mechanically interlaminated. Oligoclase, 

 labradorite, and andesite were nnited under the name of plagioclase. This 

 term has, however, been employed since to include both albite and anor- 

 thite, and in this latter sense it is generally used.* 



Tschermak, indeed, does not claim this theory to be entirely original with 

 himself, for he remarks, " Dabei verschweige ich jedoch nicht, dass die 

 Grundidee dieser Vereinfachung keineswegs neu sei und ich bemerke, dass 

 durch die friiheren Bemlihungen der Forscher, welche eine solche Vereinfa- 

 chung auf chemischer Basis ansti'ebten, also durch Sartorius von Walters- 

 hausen, Raunnelsberg, Scheerer, der Gedanke endlich so Aveit entwickelt 

 wurde, dass Andere wie Delesse, Hunt deuselben als keines speciellen 

 Beweises bediirftig hinstellten." 



Tschermak's theory was variously opposed and advocated, and on one 

 side or the other the most prominent chemical mineralogists arranged 

 themselves. It has been especially discussed by Rammelsberg, Rath, Eoth, 

 Bunsen, Petersen, Streng, and others, with the result that it is the generally 

 accepted view regarding the composition of the feldspars. 



Tschermak's theory does not appear to be well understood in England 

 or America, and although the present writer recognises his liability to also 

 misinterpret it, he deems it right to point out some of these differences of 

 opinion, believing that in the end it will lead to a more accurate conception 

 of the theory than now seems to exist. 



Streng later offered a theory for the feldspars, in which he held that they 

 were silicates, in which the Ca partly replaces the Naa, and R the Si2; 

 claiming that there were only two principal divisions; 1st, the potash feld- 

 spar, and 2d, the lime soda feldspar — the latter forming a number of varie- 

 ties with variable composition.! 



* Sitz. Wien.Akad., ISGi, 1. (2) pp. 56^-613. 



t Neues Jahr. Min., 1565, 411-43i, 513-529; 1S71, 598-61S, 715-731. 



