G. H. Williams—Cleavage in American Sphene. 489 
Cloizeaux,* between 77 is 125° 42’, so that the agreement is 
as great as could be expected in consideration of the reflections 
never being perfectly shar 
The frequent total absence of this parting in one portion 
of a crystal when it is highly developed in another part, 
as well as the fact that it is to be found parallel to only one o 
the two pyramidal faces, shows that it is not to be regarded as 
a true cleavage 
It is ieeterns easy to convince oneself by a careful in- 
lec dput is twinned accordin ng to —4R. The width of these 
bands varies Goch + to z1, of a millimeter, each one frequently 
appearing to be made up of innumerable narrower ones. ey 
often change in width and sometimes either pinch out gradually 
or suddenly disappear. No accurate measurements of the 
angle of extinction of these bands could be made on account 
of the very high refractive index of the mineral. 
There seems, therefore, to be little doubt that the apparently 
perfect cleavage, so characteristic of much of the American 
sphene is produced by a polysynthetic twinning parallel to the 
face y (= —4P Dana, —2P Des Cloizeaux, $P4 Rose), a fact 
that j co of interest as well for being a new twinning law for this 
mineral as an explanation of its characteristic parting. 
In light of the recent investigations by Miigge,t Porstner$ 
and others, on the production of secondary twinning lamellze 
by y pressure, it seems not improbable that the above described 
structure in sphene may be also due to the same cause. This 
seems the more likely since all the other minerals associated 
With the sphene from Pitcairn, in which the parting is most 
perfectly developed, show in a greater or less degree effects of 
the same kind. The calcite in which all the minerals are 
nuel de Min., i, p. 148, 1862. 
t None Jahrbuch fir Min " ete., 1883, i, p. 32; oe Seg p. 13; ib., 1884, i, p. 216. 
} Zeitschrift fiir Krys tallographie, vol. ix, 351, 1 
Am. Jour. Sct.—Tuirp Series, Vou. XXIX, ie sae ea 1885. 
33 
