MINERALOGY. 129 
of the grains of calcite in limestones, and these twin lamelle, are very 
probably due to the effects of the pressure which was exercised upon 
the rocks during their metamorphism, because Reusch had already 
shown that these lamellz could be induced in simple calcite crystals by 
slicing off two of the opposite edges of a cleavage rhombohedron, and 
exerting a gradually increasing pressure upon the little faces thus made. 
When calcite is formed in cavities and cracks of the rock, where the 
rocks had plainly taken their last form before the calcite was produced 
by their decomposition, this twinning (if it is a twinning) is not often 
found ;—a fact which might be expected if the above mentioned theory 
is correct. 
In the amygdaloidal cavities calcite sometimes assumes pretty, micro- 
scopic forms. For example: in Fig. 3 on Pl. 6 are represented the 
amygdaloids which abound in the olivine diabase at Campton falls, and 
which are filled with analcite. Before the analcite was formed, there 
was, however, a growth of hexagonal prisms of calcite, which were ter- 
minated with the planes of an obtuse rhombohedron. In some of the 
cavities these prisms had grown from side to side, thus forming a bar 
across the little chamber. 
Some of our limestones, as, for example, those at Littleton, when ex- 
amined in thin sections, exhibit peculiarities ‘in the cleavage. The lines 
are no longer straight, but traverse the grains in curves. This is prob- 
ably another result of pressure which at some time acted upon the stone. 
In some limestones the organic matters, which were originally present 
in the shells, etc., have not been entirely destroyed, but are left in a bitu- 
minous condition; and these rocks, when struck, give forth a foul odor, 
from which the stone is called fetid limestone or stinkstone. Such a 
limestone occurs at Orford. 
Numerous analyses of New Hampshire limestones have been made by 
Dr. Jackson and others.* It would be profitless to introduce them here, 
since their bearings are merely economic. They show all grades of im- 
purity, from the pure white limestone of Haverhill, which contains 99.3 
per cent. of calcium carbonate, to a gray, Cornish limestone, which con- 
tains 63.4 per cent. of impurities. Thus, by the gradual introduction of 
other minerals, limestones grade into other rocks. Besides this kind of 
* Geology of New Hampshire, Dr. C. T. Jackson, pp. 173-175. 
VOL. IV. 17. 
