Gabbroid Rocks of Minnesota. — Winclicll. 289 
seem to exist no graduations: First, large crystals from about 
one-half a millimeter to more than one millimeter in diameter, 
and sometimes exceeding one centimeter in length; and sec- 
ondly, extremely minute crystals whose presence would often 
be unobserved, but for the halos which the}- cause in the horn- 
blende. The large crystals are usually rounded and some- 
times deeply corroded (See plate XVIII, Fig. 10 and 11); 
while the minute crystals have never been seen except in well 
preservd crystals. The large crystals occur in every part of 
the rock, the small crystals are exclusively confined to certain 
areas ; and seem to represent the material derived from the 
corrosion of the large crystals. 
The large colorless crystals are often slightly clouded by 
great numbers of inclusions, which are rare or absent in the 
small crystals. The large crystals have shown in sections well 
oriented the faces :/>( 000 1 ),;//( 10 10), (i 120), (2130), (ioii),and 
probably the rare face (4371). The last face was found in a 
section strictly parallel to the vertical axis. The angle be- 
tween the prism face (1120) and the pyramidal face (4371) va- 
ried between \\° 15' and 12° 15', since the latter was not abso- 
lutely straight and regular. E. S. Dana* found a pyramidal 
face on a crystal from Paris, Me., which was likewise some- 
what irregular, and his goniometric measures varied between 
1 1° and 12°, from which he calculated that the face was (4371), 
n.aking, theoretically, an angle of 11° 56' with the face (1120). 
Cleavage parallel to the base /(oooi) is distinct, but im- 
perfect; parallel to one or more prism faces it occurs very im- 
perfectly. 
The most peculiar feature of the apatitet is its marked bi- 
axial character about iim . Careful measurements of 2c/ show 
that 2E often exceeds 10° ! 
At the same time crystals occur which seem strictly uni- 
axial. 
Crystals optically biaxial were carefully separated and 
freed from all impurities with the aid of the microscope. Mi- 
crochemical tests demonstrate the existence of potassium in 
this apatite; the test with platinic chloride gave crystals shown 
in plate XVIII, Fig. 13, and that with bisulphate of bismuth 
gave the forms shown in plate XVIII, Fig. 14. Further am- 
*E. S. Dana; hmtr. Jour. Sci. 1884. XXVI. p. 480. 
