MINERALOGY. 3/ 



that they are filled with innumerable cavities containing water. When a 

 thin cleavage piece is examined under the microscope, these cavities are 

 seen in immense numbers and of every conceivable form, two of the 

 larger of which are represented in Fig. y c and d. These cavities always 

 contain a bubble, which is not diminished in size by heating, which 

 indicates that the fluid is water. The presence of these cavities, contain- 

 ing water and a certain amount of empty space represented by the bub- 

 ble, is regarded as evidence that minerals containing them were formed 

 at elevated temperatures and pressures, since, in minerals suitable for 

 experiment, the bubbles disappear when the minerals are heated to a 

 certain temperature, showing that the bubble is an empty space formed 

 by the contraction of the fluid after the formation of the crystal. 



Fluor spar also occurs as a microscopic ingredient of some of our gran- 

 ites and sienites, as for example, on Chocorua mountain. It is recog- 

 nized in thin sections of the rocks by its very perfect octahedral cleavage, 

 and by revolving the section in a horizontal plane between the crossed 

 Nicol prisms, when it remains in every position uniformly dark. 



22. Water [H^ O]. 



Some of the purest waters in the world run in the streams and come 

 up in springs in New Hampshire. Pure spring waters are not common ; 

 but in the northern part of the state, some of the springs that come 

 through the slate rocks are well-nigh pure, and a large amount of water 

 when evaporated, leaves an inconsiderable residue. The spring waters 

 in the Dixville Notch are most remarkably pure. The reason is, that the 

 slates in these regions are composed almost exclusively of insoluble con- 

 stituents. There are, however, a large number of mineral springs in 

 various parts of the state, common in which are chalybeate waters. The 

 springs of this kind, at Amherst, Charlestown, Pittsfield, and Unity, are 

 best known. When occurring near beds of pyrites, these springs contain 

 both sulphur and iron, evidently obtained from the decomposition of that 

 mineral. Near Mt. Pleasant, just over the boundary in Maine, there is a 

 spring of this nature. 



23. Melaconite [Cu O]. 



This mineral has been formed in some places by the decomposition of 



