218 FIELD COLUMBIAN MusftuM—GEOLOGY, VOL. II. 
latter often give a striated appearance to the ‘surface of the planes, 
the direction of the strie being that of the intersection of two planes 
of the pyramid of the secondorder. In other crystals the pyramidal 
planes present a concave or otherwise irregular surface. The pyra- 
midal planes are rarely present in full number, only two or three 
occurring commonly. The usual habit of the crystals is shown in 
Fig. 5. No doubly terminated crystals were found. As a modifying 
form, the prism of the second ‘order a (1120) is common, occurring 
with planes sometimes narrower and sometimes as broad as those 
of the fundamental prism. The planes usually have a luster and 
surface like those of the first order prism. The only other modifying 
form found was a plane of the 4-4/3 pyramid (3141) found on a 
single crystal. The symbol of this plane was determined by its 
position and the following measurements: 7 
Measured. Calculated. 
it 2 WA(EO1O s) STAT eas ee 22° At a pet 
Numerous attempts were made to secure an accurate meas- 
surement of m : x (1oTo roTr), but owing to the imperfect surface 
of the latter planes success was obtained in only a single case. This 
afforded, however, a very accurate measurement, and gave the value 
49° 43’. This would correspond to a value for c : x (ooor : 10717) 
of 40° 17’. The Cerro Mercado apatite has therefore a large axial 
angle. Ordinarily, as pointed out by Palache,* apatites with large - 
axial angle are free from chlorine. A qualitative test of the Cerro 
Mercado apatite, however, made by dissolving it in nitric acid 
and adding silver nitrate, gave a precipitate of silver chloride suf- 
ficient to indicate the presence of an appreciable quantity of chlorine. 
The specific gravity of the apatite obtained with a chemical balance 
on the pure crystals gave the value 3.222. The fracture of the 
crystals is highly conchoidal. : 
Of the rocks associated with the iron ore of the Cerro Mercado, 
detailed description has been given hitherto by only two authors, 
Chrustschofff and Ordonez.{ Chrustschoff describes several vari- 
eties of quartz porphyry and quartz porphyry breccias occurring 
mostly on the eastern, northern, and higher portions of the hill. 
One of the quartz porphyries is said to contain phenacite in suffi- 
cient quantity to form practically an essential constituent. No 
such rock was noted by the present writer, and so remarkable an 
occurrence needs confirmation. Other rocks described by Chrust- 
schoff are a hornblende-bearing granite stated to occur on the west- 
* Proc. "Am Acad:, VohuxX Xx VI. presas: 
+Op. cit. 
Bol. d. Inst. Geol. de Mexico, No. 16, p. 11. 

