522 PROCEEDINGS OF THE AMERICAN ACADEMY. 



on the faces of z, for they never extend beyond the intersection of /x with 

 that face, and are absent if z is not developed. The grooves are bounded 

 by faces parallel to adjacent planes of s and of m. Very often they 

 stop short in the middle of the face as shown in figure 3. 



On fxi the markings are in the form of irregular pits or curving 

 grooves, sometimes showing approximate parallelism to the intersection of 

 m and ^ but with an irregularity giving them a character wholly dif- 

 ferent from the lines on /x. No constant difference could be observed 

 in the brilliancy of the reflecting portions of faces of the two forms, nor 

 in their relative size. Both are irregular in their occurrence on individ- 

 ual crystals, lacking nearly always some of their faces. As shown in 

 the figures, both may present on the same crystal faces of very unequal 

 size which in some cases are so large as to dominate the termination of 

 the crystal. 



The occurrence of third order pyramids in apparently holohedral 

 combination has been observed on apatite from various localities, notably 

 Knappenwand, Tyrol,* Ala, Piedraout,f and Elba.J But in none of the 

 crystals described does there appear to have been any observable dif- 

 ference between the faces of the right and left forms by which they 

 could be distinguished. 



Reference has been made in the preceding pages to striations which 

 appear quite constantly on certain faces of the apatite. They are a 

 striking feature of the crystals and the attempt has been made to repro- 

 duce them in the drawings. Their most pronounced development was 

 on the largest crystal of the collection, which is reproduced in figure 4 ; 

 the striations on the faces of z and of a were almost equally strong and 

 gave the crystal a curiously tetragonal aspect when inspected casually. 

 On both of these forms the striations are doubtless growth forms, the 

 result of oscillatory combination, on a of adjacent faces of m, and on z 

 of planes of m and y. The markings on the faces of /x and /x x seem to 

 have a different character, however. The irregularity of their develop- 

 ment, appearing on some faces as mere grooves or pits, on others 

 invading the whole face and reducing it to a dull surface, indicates that 

 they are rather the result of etching by some agent which has attacked 

 the crystals after they were formed. 



* C. Klein, Neues Jahrb. Miner., 1871, 485 ; 1872, 121. 



t G. Struever, Att. Ace. Torino, 3, 125, 1867; 6, 363, 1871; Rendic. R. Ace. 

 Lincei, Roma, 1899, 8 (1), 427-434. 



.t E. Artini, Rendic. R. Ace. Lincei, Roma, 1895, 4 (2), 259. 



