1894 THE MICROSCOPE. 163 



inclusions, large enough to be seen with the naked eye, 

 may be seen e. g., quartz in felspar ; garnet in mica ; tour- 

 maline in quartz. — Some of the quartz grains are perfect 

 little museums of other minerals. But inclosures are 

 not always minerals. Sometimes they are little spaces, 

 little lakes — lacunae as we call them — filled or partially 

 filled with fluid. Sometimes the fluid is pure water. 

 Sometimes it is water saturated with common salt or 

 other alkaline salt, and then little cubes of the salt may 

 sometimes be seen lying loosely at the bottom of the 

 cavity. Some of them, as an enterprising chemist has 

 shown, are tilled with carbonic acid in a liquid state. 

 Note the internal pressure that implies. In some, again, 

 a minute bubble of gas is seen floating along the top of 

 the fluid as in a spirit level. When you turn the crystal 

 round the bubble may sometimes be seen to move. When 

 the cavity is very minute the bubble may have a sponta- 

 neous movement, and may be seen darting like a living- 

 thing from side to side and from end to end of the cav- 

 ity, then returning to its original position, only to set 

 out on its ceaseless round once more. It is not our pur- 

 pose to go into the reasons why these inclosures are 

 present in quartz grains. It is sufficient for the end we 

 have in view that they are there, and that we learn some- 

 thing of the grain that includes them because they are 

 included in it. We may classify inclusions into several 

 groups : — 



1. Those with regular mathematical outlines, shortly knowu as regular 



(figures 2, 3, 4). 



2. Acicular, or fine needle-lifee inclusions (figures 6, 7). 



3. Spirit-level. 



4 . Irregular outlined. 



I. The regular forms are almost always crystals of 

 other minerals. Among those that come under obser- 

 vation in our investigation may be mentioned quartz as 

 an inclusion in quartz, garnet, black mica, chlorite, ru- 

 tile, kyanite, apatite, &c. It is a case of identity we 



