143 



This* figure represents a plagioclase crystal made up of the following 

 forms: P = (001), M=(010), T=(ll0), /=(110), #=(101). It is placed 



so that the prismatic zone is vertical, and the basal plane (P) inclined from 

 left to right. The obtuse edge P/M is therefore situated to the right of the 

 observer. -The planes of easy cleavage are of course parallel to P and M ; 

 there is also a cleavage parallel to T which is often sufficiently marked to 

 determine the form of cleavage flakes parallel to M. Now SCHUSTER has 

 proposed that the signs + and should be used in a conventional sense for 

 the purpose of defining the directions of extinction on the two principal 

 cleavages. The precise manner in which the signs are used is evident from 

 the figure and therefore needs no further illustration. In cleavage flakes 

 parallel to P, prepared by roughly breaking up the crystals as they occur in 

 rocks, it is not as a rule possible to determine the sign of the extinction ; but 

 in those parallel to M it is often possible, because these are freqently in the 

 form of parallelograms, the edges of which are determined by the easy 

 cleavage P and the imperfect cleavage T. On referring to the figure it will 

 be seen that when the direction of extinction, as it may be called, lies in the 

 obtuse angle of the parallelogram the sign is positive, and when it lies in the 

 acute angle the sign is negative. 



The researches of SCHUSTER have established the fact that in the normal 

 plagioclase felspars which may be regarded as isomorphous mixtures of albite 

 and anorthite, the optical and chemical characters stand in the closest possible 

 relation to each other. The specimens which have been examined both 

 optically and chemically are not perhaps sufficiently numerous to prove that 

 an absolutely continuous series exists, but they are certainly sufficient to 

 establish the existence of many felspars intermediate between the common 

 types known as albite, oligoclase, labradorite, and anorthite, and therefore to 

 render the theory of TSCHERMAK in the highest degree probable. Thus, the 

 existence of felspars between oligoclase and labradorite (andesine), and 

 between labradorite and anorthite (bytownite) which are intermediate in 

 their optical as also in their chemical characters appears to be definitely 

 established. 



Accepting, therefore, the views of TSCHERMAK and designating the albite 

 molecule (Na 2 O, A1 2 O 3 , 6 Si O,) by Ab and the anorthite molecule (2 CaO, 



