E. K Williams— Crystals of Tourmaline with Orthoclase. 275 



In the second type the rhombohedral faces, as well as those 

 of the prism, are shells, and the tourmaline does not seem to 

 decrease in thickness with its distance from the termmation. 

 After removing the shell from these combinations the enclosed 

 feldspar has been obtained having the form of a distorted tour- 

 maline crystal. 



Under this second variety there are cases in which the tour- 

 raahne prism encloses the feldspar and has no terminations. 

 The feldspar often replaces the tourma- 2. 



line, as is seen in Fig. 2, where the angles 

 between the prismatic faces are distorted ; 

 one face is found parallel to a cleavage 

 plane, usually i-\ of the orthoclase. In 

 the figure the angles afiy and fiay are 

 respectively 45° and 90° ; the tourmaline 

 prism is perpendicular to the basal plane 

 of the feldspar ; the edge toward the eye 

 has been replaced by the orthoclase, while the tourmaline makes 

 a re-entering angle behind it at the arrow-point 

 is the cleavage plane parallel to /of the feldspar, 

 are readily separated from their feld spathic 

 matrix, and cleave parallel to its basal plane ; 

 each section thus given shows an uneven 

 shell of tourmaline that in places entirely 

 disappears. In all the cases mentioned the 

 cleavage of the enclosed feldspar is parallel 

 to that outside. 



Fig. 3 shows where the tourmaline has 

 been deposited on a cleavage face of the 

 feldspar, and covers about one square inch ; 

 the vertical edge toward the observer is rounded, and presents no 

 measurable angle ; the plane ahcd is a cleavage plane parallel 

 to i-l of the feldspar. In one instance the feldspar inside the 

 tourmaline encloses another shell of tourmaline filled with or- 

 thoclase. 



The few specimens thus far obtained seem to point to simul- 

 taneous crystallization. There is no law governing the relations 

 in position of the two minerals ; for, in the same mass of matrix, 

 there are crvstals of tourmaline whose vei-tical axes make with 

 the basal plane of the orthoclase angles varying from to 90 . 

 borne of these enclose the feldspar in prisms whose angles cor- 

 respond exactly with those of theory, while others are distorted 

 and are found between the cleavage planes of the feldspar. A 

 more extended studv of these interesting forms, aflorded by a 

 greater number of specimens, may throw more light on this 

 smgular combination. 



Philadelphia, Dec. 28, 1875. 



