TEliTTARY VOLCANIC ROCKS-LATITE-PHONOLITE. 
75 
rocks the apatite crystals are crowded with minute inclusions, which probably give 
the smoky color to the mineral in the hand specimens. A high power reveals consid¬ 
erable as to the nature of these inclusions. Some are approximately equidimen- 
sional and rarely reach the maximum diameter of 0.001 mm. From this shape they 
pass by all gradations to the most common form, that of exceedingly slender rods of a 
maximum observed length of less than 0.01 mm., and a cross section which is often no 
more than 0.00002 mm. in diameter and perhaps less. Their distribution throughout 
the crystals is not uniform. Near the prism faces they are always crowded and near 
the terminal planes they diminish in number to almost complete absence. Within 
the body of the crystal they mav be very evenly disposed or on the other hand be so 
crowded about certain lines or along planes that portions of the crystal sometimes 
become opaque. Red or black irregular compact grains of small size but of much 
greater dimensions than those of the inclusions can often be seen, near the edges, to 
be thus built up of innumerable inclusion individuals so closely packed that they have 
coalesced. The larger or stouter forms are distributed mainly along lines or planes, 
which may he slightly curving and which from all appearances have no relation to 
crystallograpliic directions. 
In spite of this lack of uniformity of distribution as regards number and size, 
they have a definite crystallographic arrangement or orientation. The larger inclu¬ 
sions can be seen to possess in general verj* perfect negative crystal form, though 
some are irregular. The slender forms have their long direction parallel to the 
vertical axis and are arranged in rows parallel to the three other axes. 
Great numbers of the inclusions, particularly the larger, stouter ones, have a 
much lower refractive index than the apatite, and the width of their black borders 
suggests that they are fluid inclusions. This idea is confirmed by the presence, in 
many, of a minute round particle with a still broader black rim which can in some 
cavities be seen to be in constant motion and to be influenced by gravity and which 
is beyond question a gas bubble. Some cavities contain, in addition to the liquid and 
the gas, a tiny angular speck which is probably a salt crystal separated out from the 
liquid. 
These inclusions, when obviously fluid, range from nearly or quite colorless 
• through light yellows and reds to brownish yellow, brownish red, or deep red. Many 
of the long and very narrow ones seem to be composed of a solid material, which 
is sometimes red and sometimes black and opaque, with all gradations between. 
Examination of aggregates of these solid inclusions shows that they are composed of 
hematite. 
The foregoing facts appear to justify the following conclusion: At the time 
of the formation of the apatite—that is, near the beginning of the crystallization of 
the magma—water was present in addition to the chlorine or fluorine, or both, of the 
apatite, and it held ferric iron and possibly other bases, in solution. Later a change 
of conditions caused a precipitation of part or all of the iron as hematite, and the 
small solid grains within the liquid may be crystals of a different material. The 
yellow colors may be due to more or less concentrated ferric solutions, maintained 
possibly by pressure or by a small amount of some reagent, such as chlorine. 
•Depending on the amount of hematite which has been deposited on the walls of the 
cavity, the liquid appears more or less red or the inclusion seems to be wholly solid. 
