MINERALOGY. 123 
revolved on the stage of the microscope, while only the lower Nicol is 
on the instrument, sections not parallel to the base show a greater 
absoption of light in one direction than they do in the other. 
In our eruptive rocks apatite is very abundant. It is usually micro- 
scopic, and only visible in thin sections. It is apt to be aggregated in 
some parts, while other parts of a section show none. Fig. 4 on Pl. 6 
represents apatite as it appears in the diabase at Bemis rook. It will 
be noticed that the needles, though small, pierce through all the other 
minerals. Such little apatite needles as these are apt to be seen in almost 
all our rocks. : 
But apatite, as a rock constituent, reaches much larger proportions 
without becoming macroscopic. For example: the augite sienite of Jack- 
son is filled with very perfect crystals which are large enough for optical 
examination. Their basal sections remain black when revolved between 
crossed Nicols, while the prismatic sections are black when the long axis 
is parallel with the plane of vibration of the light. Again: the gabbros 
at Waterville and Mt. Washington contain apatite in fine crystals of 
some size, but which first become evident in thin sections, and which 
offer some interesting peculiarities. Fig. 6 on Pl. 8 is a representa- 
tion of the apatite as seen in this rock. It will be noticed that the 
crystals have taken form and position without reference to any other 
constituent. They pierce the infusible olivine and magnetite as readily 
as the pyroxene and feldspar. It is the only substance that has crystal- 
line outlines. Some of these crystals are crowded full of minute cavi- 
ties. These cavities are heaped more abundantly in the centre of the 
crystals. Such crystals have been observed by Zirkel, Rosenbusch, and 
others. Again: some of the crystals appear to have other crystals run- 
ning through them. Sometimes there is but one large one, and again 
there are several. The sides of the interior crystals are parallel with 
those of the large one; but perhaps of more interest is the odd outline 
of many crystals, which in part are bounded by straight crystalline edges, 
while the remainder of the crystal is jagged and rounded, and bears all 
the appearance of having been eaten into by some reagent. Sometimes 
the whole half of a crystal looks as if thus dissolved, and sometimes 
merely a piece of the margin is destroyed. Effects of this kind have 
been observed in augite and hematite crystals in basaltic rocks, and they 
