MINERALOGY. Ill 



but when stanrolite sinks to microscopic proportions in the slate rocks 

 of the Connecticut valley, it shows what is rarely seen, — really simple 

 crystals. The color of this microscopic staurolite is deep brown. It is 

 highly dichroic, and if its sections are revolved over one Nicol, when the 

 long axis of a crystal is parallel to its plane, the crystals are deep brown, 

 and when at right angles thereto, they are nearly white. The crystals 

 are extremely impure. They possess no terminations ; but the direction 

 of the prism is indicated by their fibrous character, and they are recog- 

 nized as orthorhombic, since between crossed Nicols the maximum 

 extinction of the light in all sections takes place when this striation is 

 parallel to the plane of one Nicol. Basal sections do not show the stria- 

 tions, but are still dichroic (distinction from biotite), and show some very 

 rude attempts at the formation of a six-sided figure, which is more 

 often nearly round. None of these microscopic crystals are twins. They 

 might at first be mistaken for biotite, but not after a moment's examina- 

 tion. The appearance of this common staurolite is given in Fig. 4 on 

 PI. 8. It is drawn as it appears when the lower Nicol is on the instrument, 

 and the plane of the light is indicated by the arrow. The excessive 

 impurity of the staurolite will be noted ; but this character is much more 

 evident when both Nicols are upon the instrument. This figure is intro- 

 duced as explanatory of the analysis with the high percentage of silica, 

 and also as showing a deceptive, simple form into which staurolite sinks 

 in our rocks. 



Hydrous Silicates. 



New Hampshire, which, as is well known, is mostly occupied by gran- 

 ite hills and old crystalline rocks, is mostly composed of anhydrous sili- 

 cates, and, as might be expected, is poor in the hydrous species, for there 

 has not been the opportunity here for their formation. The gre^t trap 

 dykes of the Mesozoic, which have proved to be so rich in these species, 

 come only to our boundaries; and, though trap dykes are abundant in 

 our state, they are not large, are rarely amygdaloidal, and do not offer 

 conditions favorable to the formation of minerals from their decompo- 

 sition: hence our list of hydrous silicates is small in comparison with 

 the number of species described. Those which occur in any abundance 

 belong to the foliated micaceous margarophyllites. 



