486 F. EUTLEr ON SOME PECULIARITIES IN THE 



the conclusions at which I arrive from the observations now described 

 may be briefly summed up as follows : — 



i. That massive varieties of orthoclase often exhibit a cross- 

 hatched striation, as already noticed by many observers, but that 

 these striations do not represent all the structural planes directly or 

 indirectly demonstrable which internally traverse the cleavable 

 mass. 



ii. That this structure is not yet satisfactorily accounted for, being 

 sometimes accompanied by strongly chromatic effects, suggestive of 

 twinning, under polarized light, while at other times under similar 

 circumstances the same structure appears to be represented merely 

 by divisional lines which do not offer any differences of colour such 

 as one would expect from a twinned structure of the mineral. 



iii. That this structure is often visible in patches imbedded in 

 felspars which merely show parallel banding. 



iv. That both in such patches and in some definitely developed 

 crystals the cross-hatched structure may extend over only a very 

 limited area, and that the remaining portion of the patch or crystal 

 may only exhibit parallel banding or no banding at all. 



v. That, in composite instances such as these, we are as yet 

 unable to determine by microscopic means the system to which such 

 a crystal or patch should be referred. 



vi. That there is a structure developed in the sanidine crystals of 

 some trachytes analogous to a structure to be found in the crystals 

 occurring in the Mexican obsidian just described, some of these 

 crystals appearing to be also identical with some crystalloids formed 

 in a furnace-slag which was examined and described by the late 

 Hermann Vogelsang. ; 



vii. That the sanidine crystals just alluded to are twinned and 

 may be regarded as true crystals, and that, if so, there is but little 

 distinction to be made between some crystals and crystalloids. 



viii. That the crystals or crystalloids in the Mexican obsidian 

 show great discrepancy in the angular intersections of their internal 

 striae or divisional planes, but that this does not invalidate the as- 

 sumption that they belong to the same species, since a drop of 

 solution of chloride of ammonium evaporated on a glass slip will 

 show a variation of angles from about 72° to 90°. 



ix. That the present method of discriminating between monoclinic 

 and triclinic felspars answers sufficiently well for ordinary purposes, 

 but that it is often inadequate for the determination of doubtful 

 examples, and that such examples are of more frequent occurrence 

 than one would at first be led to suspect. 



x. That the estimate of the number of twin lamella? occurring in 

 crystals of plagioclase, as given by some observers, is liable to error, 

 the obliquity of some sections tending to augment the apparent 

 number of the lamellae by the overlap of adjacent plates which 

 polarize in complementary colours. 



