116 S. L. Penfield — Mineralogical Notes. 



basal sections gave with the polarizing microscope extinctions 

 inclined about 38°-10° from the edge formed by the meeting 

 of the two cleavages. These optical relations seemed entirely 

 unlike those of oligoclase and the material was therefore de- 

 scribed as abnormal. Offret" has since shown that the Bakers- 

 ville oligoclase has normal optical properties and, therefore, in 

 order to reconcile these differences and to make a further study 

 of the material a new supply was secured from Mr. Geo. F. 

 Kunz of New York and Mr. W. Yance Brown of Plum Tree, 

 ~N. C, to whom the author's sincere thanks are due. Among 

 the specimens some pieces presented the normal cleavage, 

 polysynthetic twinning and optical properties of oligoclase. 

 Some, however, that were clear and free from twinning, 

 showed the abnormal properties as previously described, but 

 from the examination of a number of fragments it was soon 

 discovered that the second cleavage is not parallel to (010) and 

 that the feldspar thus presents an unusual and unexpected 

 cleavage or parting instead of abnormal optical properties. 



In oligoclase the angle 7, between the a and h axes is prac- 

 tically 90° and the extinction diiection on basal sections is +1° 

 or almost parallel to the a axis, hence it follows that the angle 

 of 38°-40° which the cleavage makes with the axis of greatest 

 elasticity in a basal section serves to orient the cleavage with 

 reference to the lateral axes. From the measurements of vom 

 Kathf it has been calculated that the pyramid (121) makes 

 angles of 92° 35' =(001 A 121) and 87° 25' with the base, the 

 measurement of the abnormal cleavage being about 88°, and 

 that the trace of the pyramid on the base intersects the edge 

 between (001) and (010), or the direction of the a axis at an 

 angle of 38° 1', about corresponding to the extinction direc- 

 tion. Exact measurements cannot be made for determining 

 the position of this peculiar cleavage, owing to the poor re- 

 flection which it gives and the absence of other planes except 

 the basal cleavage, but it is distinctly parallel to a pyramid 

 and the form (121) has the position to yield all of the ab- 

 normal properties that were observed. This abnormal cleav- 

 age is well shown on many of the specimens and whether it is 

 a true cleavage or a parting of secondary origin, produced 

 perhaps by pressure, is uncertain but it is probably the latter. 

 The forms that can be broken out, bounded by the basal and 

 abnormal cleavages and an irregular fracture inclined to the 

 base at an angle about equal to t 3 of the feldspars, look like 

 ordinary cleavage blocks of feldspar, and thus caused the 



* Bull. Soc. Min. de France, xiii, p. 648, 1890. 



f Pogg. Ann., cxxxviii, p. 464, 1869. Also Dana's Mineralogy, Sixth edition, 

 p. 332. 



