Fig. 141. 

 Basal section of 



A mmonium-lithium- sulphate. 



If an individual be studied composed of several intergrown lamellar 

 crystals (fig. 141), local triangular spots are met with, consisting 



of lamellae interwoven at angles of 

 60, which become extinguished between 

 crossed nicols in any situation ; but these 

 are just the parts endowed with rotatory 

 power. There cannot be the least doubt 

 here, that the rotatory power of these 

 parts is governed by the crossing of the 

 biaxial laminae, in full accordance with 

 Mallard's theory. 



In general it has become clear from 

 these researches, that a great number of 

 crystals showing optical rotatory power 

 also exhibit optical anomalies in the 

 sense indicated above, and that these 

 dextro-, or laevogyratory uniaxial crystals, are in reality all very 

 complicated twins of lower symmetrical material. They are there- 

 fore true pseudo-symmetrical crystals, built up according to definite 

 twinning-laws, by a great number of biaxial lamellae, which in an 

 analogous way to that demonstrated in the cases of phillipsite and 

 chabazite, combine into an apparently higher symmetrical, "mimetic" 

 aggregate. The special circumstances of crystallisation seem to have 

 a certain influence on the arrangement of the composing lamellae, 

 so that within certain limits a fluctuation of the optical properties 

 of such crystals may evidently occur. The inconstancy of the 

 magnitude of the rotatory power of such crystals was in many 

 cases confirmed by direct observations. 



9. Now Mallard, basing his theory on the observed fact that 

 the crystals which show optical anomalies are just those whose 

 geometrical properties are closely approximate to those of higher 

 symmetry, considers the optically anomalous crystals as without 

 exception pseudo-symmetrical aggregates of lamellae, the space-lattice 

 of which has a lower degree of symmetry than the crystal as a 

 whole possesses. l ) If for instance (fig. 142) a rhombic crystal has a 

 space-lattice, the layers of which, parallel to the plane of drawing, 



1) E. Mallard, Explication des Phenomenes Optiques Anomaux dans les 

 Substances Cristallisees, Paris, 1876; Ann. des Mines (7). 10. 60. (1876); Bull, 

 de la Soc. Miner. 5. 144, 214. (1882); 7. 349. (1884); A Scacchi, Zeits. der deut- 

 schen Geol. Ges., (1864), p. 35. 



