OLDER PERIDOTITES OF SCOTLAND. 



365 



temperatures * ; and, finallj^, Klein has demonstrated that leucite 

 loses its twin lamellae, and exhibits the isotropism of cubic crystals 

 at a very high temperature f. 



The conclusion to which all these interesting results point is, that 

 the properties of a crystal formed at any given temperature may be 

 greatly modified when that temperature is altered. Thus leucite, 

 while contracting in cooling from the temperature at which it was 

 originally formed, becomes subjected to internal stresses and strains, 

 resulting in lamellar twinning, and even in that alteration of the 

 angular measures of the crystal so long ago pointed out by Yom Eath. 

 Such being the case, the question which cannot fail to present itself 

 to petrographers is : May not the twin-lamellae of the plagioclase 

 felspars, like the similar structure in calcite, be not an original and 

 necessary character of the crystals, but one developed in them by 

 pressure or strain ? 



There are not a few facts which support the hypothesis we have 

 just enunciated. Lamellar twinning is often altogether absent, as 

 we have already seen, in plagioclase felspars ; and in the case of the 

 rocks we are now studying, such untwinned plagioclase crystals are 

 especially found in certain veins. In other cases the lamellar twin- 

 ning is found affecting certain portions of the crystal only, and in 

 the most conspicuous manner. The twin-lamellae, too, often stop 

 suddeuly, or are replaced by narrower and more numerous, or 

 broader and less numerous ones, as is the case with lamellae arti- 

 ficially produced in calcite. Like the twin-lamellae in the rock- 

 forming calcite also, those of plagioclase-felspar are sometimes bent 

 in the most remarkable manner. Sometimes the same crystal is 

 found exhibiting two series of lamellar twinnings intersecting one 

 another, as is also the case with calcite. Perhaps the most striking 

 and significant circumstance of all is that where a felspar-crystal is 

 seen to be bent or compressed between other crystals in a rock, 

 the part of the crystal so affected will be traversed by twin-lamellae, 

 from which other parts of the crystal may be altogether free J. 



That the tendency to that peculiar disposition of molecules which 

 constitutes lamellar twinning must exist from the first both in 

 calcite and in plagioclase felspars is, of course, freely admitted ; but 

 the rearrangement of these molecules in obedience to certain laws 

 seems to be determined by the application to them of external 

 mechanical forces. In great, slowly- cooling masses of rock, like 

 those in the centre of volcanic cones, composed of minerals having 

 different coefficients of expansion, not only in different crystals but 

 along different directions in the same crystal, we have forces at 

 work competent to produce those strains and stresses which have 

 beeli proved to be capable of inducing such a rearrangement of the 



* Neues Jahrb. fiir Min. &c. 1884, i. p. 193. 



t Nachrichten der Kon. G-es. d. Wiss. Gottingen, May 1884. 



\ Admirable, illustrations of some of these appearances will be found in 

 Cohen's 'Sammlung von Mikrophotographien zur Veransehaulichung der 

 mikroskopiscben Structur von Mineralieu und Gesteinen,' taf. xxx. 2, 3, Ixxv. 

 1, 2, 4. See also xxxi. 1, 4, and xxx. 4. 



