480 ^F. KUTLEY ON SOME PECULIARITIES IN THE 



mediate between the directions of the rectangular striations. Now 

 it is evident that lamellae and other inequiaxal bodies do not assume 

 definite directions either in rocks or in minerals without some good 

 cause ; and in the present case I think these coarser lamellae or 

 crystals may be taken to evince the presence in this mineral of 

 structural planes other than those indicated by the cross-hatched 

 striation. As this section corresponds in general respects with other 

 sections of orthoclase which I have cut parallel to the basal cleavage, 

 and as, besides the observers already cited, Rosenbusch and others 

 state that these striae run, the one set parallel, and the other at 

 right angles to the edge formed by the faces OP and ooP'oo , it seems 

 that these coarser plates or crystals lie in directions parallel to the 

 ooP planes ; for although the angles 105° and 75° do not agree with 

 the angles formed by the ooP planes in orthoclase, yet the discre- 

 pancy may be accounted for by the oblique direction in which, if 

 really parallel to ooP, they are here viewed, assuming that the 

 plane of section coincides with the basal plane, in which case the 

 divergence of the plane of section from a plane at right angles to 

 ooP would be 22° 16', since the observed inclination of oP on ooP 

 is 67° 44'. Such an obliquity in the direction of the plates to the 

 direction of vision would suffice to augment one pair of alternate 

 angles some 10 or 15 degrees, and to proportionally diminish the 

 other pair. And, as Hosenbusch records an instance of lamellae of 

 albite assuming a similar direction, it seems highly probable that 

 further research will give us more evidence of some structure in 

 orthoclase parallel to the ooP faces. Several observers have ex- 

 pressed the belief that the cross-hatched structure is connected with 

 the rectangular cleavages of orthoclase ; but from an examination of 

 the broken edges of several sections in which this structure is shown, 

 I have found remarkably perfect cleavage in one direction, but most 

 unsatisfactory indications of it in the other — a discrepancy far 

 greater than one would have anticipated from the well-defined 

 cleavages which occur in both directions in hand-specimens. The 

 accidental fractures on the edges of very thin sections, however, are 

 scarcely to be regarded as of much importance, except in cases where 

 cleavage is very perfect and easily produced. One of the points which I 

 especially wished to ascertain was whether the cross-hatched striation, 

 just alluded to, was ever partially developed ; for this purpose it ap- 

 peared best to examine small imbedded crystals. Pig. 2 Plate XXIII. 

 represents a crystal occurring in perlite from Schemnitz. As is 

 rather common in crystals imbedded in perlites and pitchstones, 

 the angles are somewhat rounded. Upon one side, extending over 

 a limited area, and upon one side only, is the cross-hatched striation 

 visible in this crystal. Lines of accretion are shown in other parts ; 

 but the cross-hatching is only partial. It may be objected that we 

 do not know the direction in which this crystal is cut — that if cut 

 in some other direction it would show the cross-hatching throughout. 

 I very much doubt it. To obviate the difficulty of such an objection, 

 the section, part of which is shown in fig. 5, Plate XXIII., may be 

 referred to. This section (magnified 115 diameters) is cut from a 



