MINERALOGY. 37 
that they are filled with innumerable cavities containing water. Whena 
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. 7c and @. 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. MeraconiTE [Cu O]. 
This mineral has been formed in some places by the decomposition of 
