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 PI. 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 PI. 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 



