MINERALOGY. 1 29 



of the grains of calcite in limestones, and these twin lamellas, 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 lamella5 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 o/ New Hampshire, Dr. C. T. Jackson, pp. i73-i75- 

 VOL. IV, 17. 



