OLDER PERIDOTITES OF SCOTLAND. Oi i 



From these iuclusions, -srhich are of such minuteness as oul}- to 

 be imperfectly seen with the highest powers of the microscope, it is 

 easy to make the transition to others which are absohitcly ultra- 

 microscopical, but which can be detected by the effect which they 

 produce on the light-waves that traverse the crystal. 



In this way we are led to the understanding and interpretation 

 of another peculiarity possessed by the felspars in the most deeply 

 seated masses, which is especially worthy of notice. Such felspars 

 not unfrequently display the vhatoycuit effects so characteristic of 

 some of the labradorite from Labrador, while the felspars formed at 

 less depths never exhibit this peculiarity. 



According to Breithaupt, the labradorite exhibiting a play of 

 colours has a different density from the varieties without that 

 peculiarity ; and Yon Bonsdorf has shown* that while the former has 

 a percentage of silica of 57, the latter has one of only 52. 



The researches of Eeuschf, Schraufi, and many other investigators 

 have shown that this ])jay of colours is due to a series of thin plates, 

 developed along certain planes within the crystal. These plates 

 appear to be of ultra-microscopical dimensions, but by producing 

 interference give rise to the exquisite play of colour exhibited by 

 brachydiagonal sections of the crystal when held in certain positions. 

 Although it is impossible to trace the structure to which this 

 peculiarity is due by means of the microscope, yet the circumstance 

 of its being exhibited only in the felspar of deep-seated rocks is of 

 great significance when considered in connexion with the other 

 phenomena which we have just described. 



By the study of a large number of examples, it is clearly seen that 

 these changes are quite independent of the passage through the 

 crystals of water from the surface, which produces kaolinization, and 

 sometimes leads to the penetration of serpentinous and other decom- 

 position-products, along lines of fissures into the interior of the felspar 

 crystals. These and other important alterations which have been 

 superinduced in the felspar-crystals of these deep-seated rocks, sub- 

 sequently to their original formation, we hope to discuss in a future 

 paper. The difi'erent changes we have been describing, like the 

 analogous ones in the pyroxenes and olivines, are, however, clearly 

 related to the depth from the surface at which the rocks were 

 originally situated, the greatest change being in every case displayed 

 by the most deeply seated rock-masses. 



The Pyroxenes. — Both the monoclinic pyroxenes (augites) and the 

 rhombic pyroxenes (enstatites) exhibit in a very striking manner 

 the effects of alteration when they form parts of rock-masses 

 originally situated at great depths from the surface. 



By the old German miners the name of " Schiller-spar " was 

 given to those mineral substances which exhibit a " Schiller " or 

 sheen, ^. e. a submetallic reflection when the crystal is held in 

 certain positions. Freiesleben and the early German mineralogists 



* Jahrb. fiir Min. &c. (1838) p. (581. 



t Poggend. Ami. vol. cxvi. &c. 



+ Sitzungsb. der k. k. Akad. Wien, vol. Ix. (1869). 



