306 
MINERALOGY: G. P. MERRILL 
torn of the section. No satisfactory interference figure is obtainable 
from this section, though a very indistinct dark brush sweeping across 
the field indicates that the plane of the optic axes cuts the obtuse angle 
of cleavage. 
In figure 5 is shown an occurrence in the Mocs stone concerning 
which there was considerable doubt, not merely on account of the per- 
fection of the cleavage but from the fact that it showed an indistinct in- 
terference figure in which the axial angle was so small that it was at 
first thought to be uniaxial: it was, moreover, optically negative, and 
more nearly resembles normal apatite than do the other occurrences 
under discussion. To settle the matter the cover was removed from the 
slide and the mineral treated with a drop of nitric acid as in previous 
cases. Within an hour from the time the drop was appHed the mineral 
was entirely eaten out and the solution, as before, yielded lime and phos- 
phoric acid, with the proper reagents. 
The accumulated determinative characteristics of the doubtful min- 
eral so far as made out by the various workers are then as follows : 
Colorless, with but few inclosures, two cleavages, imperfect and in- 
terrupted, cutting at an acute angle of 60°; optically biaxial and posi- 
tive (?), with large 2E; birefringence weak, less than 0.005, refractive 
indices, as determined by Dr. Wright, a = 1.623 ± 0.002 and 7 = 1.627 
=t 0.002; no pleochroism, and often with undulatory extinction; polari- 
zation in light and dark colors and at times almost completely dark 
during a revolution of the stage between cross nicols; easily soluble in 
cold nitric acid, giving solutions reacting for lime and phosphoric acid; 
very brittle and occurrence sporadic. 
There is apparently no question then but that we have a heretofore 
unrecognized form of mineral phosphate as a fairly common constitu- 
ent of meteoric stones. From normal apatite it differs in its low bire- 
fringence as well as cleavage and general optical characters. From fran- 
colite it would appear to differ in that so far as determined it is optically 
positive, while the latter is negative. Just how much weight is to be 
attached to this last characteristic, is yet to be determined. One has 
a natural hesitation in relegating it to francolite for genetic reasons, 
since francoHte, so far as known, is a secondary mineral and a deposit 
from aqueous solution. It is to be noted, however, that the mineral 
I have been describing occurs with outlines suggestive of its secondary 
origin and filling drusy cavities formed at the time of the original con- 
soKdation of the stone in which it occurs. Whether secondary or not, 
as the term is usually employed, it is certainly a product of the latest 
period of crystallization. Until the material can be found in such quan- 
