ON PYROXENIC COMPONENTS IN CERTAIN VOLCANIC ROCKS. 85 



each other. These processes are not only peculiarly curved but some- 

 what rolled up along a certain axis, so that the entire shape often does 

 not come out at once at one focal distance. However these bodies 

 normally have their branches and processes extended along one plane. 

 Their form usually assumes a rhomboidal outline as represented in 

 fig. 15, 17, 18, or rarely disymmetric as in fig. 16. The form 

 represented in fig. 18 reminds us of the clinopinacoidal section of 

 Augite. Indeed it has been found that some of these well formed 

 microlites extinguishes the light obliquely, the direction of extinction 

 making 1 40° with the longer sides of the rhomboidal figure. It is to 

 be noted that the smaller forms hardly exhibits an interference-colour. 



A very interesting phenomenon which has been observed with 

 regard to these skeleton-crystals is, that they are very frequently at- 

 tached to the crystals of the Rhombic Pyroxene in a very regular man- 

 ner. It is found that the crystals of the Rhombic Pyroxene, when looked 

 at upon the macropinacoidal face a = oo P 56 , have the angles and edges 

 of the terminal pyramidal faces (i or e) set with several curved spines 

 arranged in a very symmetrical maimer. Fig. 3, 7 — 9, show such 

 crystals. It will be found that at the solid angles of the pyramidal faces 

 these spines are much larger than elsewhere and sometimes branching. 

 They are always disposed in such a way that they all lie parallel to the 

 macropinacoidal face a. Thus when these crystals are looked at upon 

 the brachypinacoidal face b = cc P cc , these spiny processes appear 

 all running in one direction as in fig. 6, 9. 11, i. e.. parallel to the face a. 

 There is no doubt as to the identity of these spines with skeleton-crys- 

 tals of Augite alreadv described, so that the regular attachment of the 

 spines to the crystals of the Rhombic Pyroxene can be explained by the 

 tendency which the Augite has to form parallel-intergrowth with the 

 latter. The narrow bands of Augite attached to the crystale of the 



