May, 1904. GEOLOGY OF WESTERN MEXICO—-FARRINGTON. 217 
the martite crystals to prove that they were derived from pyrite 
and Chrustschoff remarks an entire absence of pyrite. Yet, der- 
ivation of the martite from magnetite seems equally unverified, 
since crystals of magnetite from the mountain have not yet been 
positively identified, and the martite contains only a trifling quantity 
of Fe O. The octahedral form shown by the martite crystals is 
to be sure more common to magnetite than to pyrite, but it is not 
uncommon to pyrite. 
It seems uncertain as yet, therefore, whether the martite origi- 
nated from pyrite or magnetite, or whether indeed it may be a dimor- 
phous form of hematite. Still, the weight of evidence seems to be 
in favor of its origin from magnetite. 
The apatite crystals collected by S. E. Meek (Mus. No. M 7668) 



Fic. 5. Apatite. Cerro Mercado. 
were obtained on the northern side of the mountain at a single locality, 
where they occur loose in a sandy soil either free or implanted upon 
or imbedded in pieces of quartz. The pieces of quartz also lie 
loose in the soil and are evidently derived from the breaking down of 
some older and larger mass. The quartz is usually chalcedonic in 
character, but in some specimens shows slender crystals a few milli- . 
meters in length. The apatite crystals are prismatic in habit, and 
range from 2cm.in length down. They are transparent except where 
coated with iron-oxide or silica. Their color is that of the true 
asparagus stone, viz., pale greenish to honey-yellow. On heating a 
crystal or fragment, it at first turns darker yellow in color, 
but at a higher temperature all color gradually disappears, and is 
not regained on cooling. The dominant planes on the crystals, and 
those constantly present, are the prism of the first order, m (1oTo), 
and the unit pyramid x (1011). Of these the prismatic planes are 
usually flat and bright, but the pyramidal planes present a cracked 
and often irregular surface. This cracked appearance is found on 
examination to be due to a cleaving or parting parallel to the pyrami- 
dal surface, broken in turn by other cleavages at right angles. The 
