272 Prof. J. P. Cooke on the Vermiculites. 



does produce the variation in certain cases ; and it must remain 

 for future investigation to assign the limits of the influence which 

 this cause may exert. We would only remark in conclusion that, 

 although in the fifty-seven varieties of mica examined by Senar- 

 mout he did not note a single instance in which the position of 

 the plane of the optical axes (with reference to the diagonal of the 

 rhomb) was different on different parts of the same specimen, or 

 even on different specimens from the same locality, he does de- 

 scribe and figure several remarkable macles of Muscovite mica 

 similar to those of vermiculite described above and represented 

 by fig. 10. In one instance the plane of the optical axes is 

 parallel to the shorter, and in the other to the longer diagonal 

 of the rhombic prism ; but in both cases it has the same relative 

 position in the several individuals of the macles, the three planes 

 forming with each other angles of 60°. On the plate of another 

 made a difference of optical angle was observed on different por- 

 tions of the plate ; and this effect was probably similar to that we 

 have been studying in this paper. On another plate from the 

 same crystals, he observed a superposition of the laminae of the 

 different individuals of the made ; and the following language 

 by which this phenomenon is described, and which is the only 

 reference made to it, shows how closely he came to the results 

 recorded in this paper : — 



"Une partie seulement du cristal made est commune aux 

 trois lames ; mais ici certaines plages (pointillees sur la figure) 

 ont un caractere optique tout particulier ; dies ne cessent jamais 

 de developper des couleurs dans la lumiere polarisee, quelle que 

 soit d'ailleurs ^orientation de la lame cristallisee. II est evident 

 que dans ces plages il y a superposition de lames appartenant a 

 des cristaux orientes, les uns comme le petit, les autres comme 

 le grand cristal, de facon que quand les unes ont leurs sections 

 principales dans le plan de polarisation, les sections principales 

 des autres font avec ce plan des angles de 60 degres. 



"Cet exemple prouve done, non-seulement que les micas 

 peuvent se grouper lateralement sans que leurs clivages cessent 

 d'etre paralleles, mais que des feuillets superposes peuvent meme 

 appartenir a des cristaux dont Porientation differe de 60 degres. 

 Un pareil mode de groupement, qui ne trouble ni la regularity 

 de la cristallisation, ni la transparence, semble indiquer que 

 Parrangement moleculaire de ces prismes rhomboidaux differe 

 tres-peu de celui qui conviendrait au prisme hexagonal regulier." 



