and their Contact Zones. 35 



and lime, or baryta. The felspar in which the alkali is 

 potash, is as orthoclase monoclinic, while as microcline it is 

 triclinia The felspar in which the alkali is soda is only 

 known in its independent form as the triclinic albite. But 

 the admixture of soda which is constantly found in varieties 

 of orthoclase, such as adularia, in which it evidently does 

 not exist as an intergrowth of albite, proves that in these 

 cases it must be present as an iso-morphic mixture, and we 

 have then the soda felspar albite in its monoclinic form. 

 Similarly the small percentage of lime in orthoclase, or of 

 potash in albite, or in anorthite, points to iso-morphic admix- 

 ture of the potash, soda, and lime felspars. The baryta 

 felspar is not known in independent existence, but hyalo- 

 phane "is a mixture of potash felspar with that barium 

 compound which in its composition is the analogue of the 

 lime compound anorthite."* 



These facts indicate that the whole of the felspars are 

 an isodimorphic series. Such series are not rare in nature. 

 I may instance. the anhydrous carbonates of the iso-dimorphic 

 calcite and aragonite groups. 



Amvhibole. — Throughout the whole area examined am- 

 phibole in some variety forms an essential constituent in the 

 majority of the examples which I have collected ; but in the 

 central and southern parts of the great intrusive mass it 

 occasionally predominates to the exclusion of almost every 

 other mineral. In forty slices which I have taken as repre- 

 senting the general composition of the intrusive dioritic 

 masses, I find that it occurs as an essential constituent in 

 nineteen. In two of these it forms almost the whole bulk 

 of the mass ; in six it is merely accessory to the magnesia 

 mica, and in five it is absent ; in three cases only does it 

 occur in rocks devoid of free quartz. I find it in these 

 examples to be associated with the felspars of groups 1 

 once, 2 seven times, 1 and 2 ten times, 3 and 4 nine 

 times, and 1, 2, 3, and 4 once — thus showing a slight preference 

 for the lime and soda lime felspars rather than the potash 

 felspars. 



Amphibole occurs in more than one variety. In the 

 granitic quartz diorites it is in more or less ill-defined, short, 

 prismatic crystals, or in irregularly shaped plates ; in those 



* Tabellarische ilbersicht der Einfachen Mineralien, &c. P. Groth, Braun- 

 schweig, 1874. I am under many obligations to this admirable review of 

 the mineral kingdom. 



