MINERALOGY. IO9 



part ill the reaction, sphene is formed. The dioritcs and amphibolites 

 about Littleton are rocl<;s in which sphene is common. 



Again : on the road from the Glen house to the top of Mt. Washing- 

 ton, some large dykes of diabase occur. They are full of titanic iron. 

 Now, on the junction of these dykes with the surrounding rock, a thick 

 layer of a very ferruginous chlorite occurs, which is a product of the 

 decomposition of the trap. This chlorite is filled with crystals of sphene. 

 This association of minerals is common, and perhaps may often be ex- 

 plained in this way. 



69. Staurolite [H^ (Mg, Fe)^ AP' Si' O'^]. 



Staurolite is very common in our slaty rocks. Here as elsewhere it 

 is found in the twin crystals, from which it derives its name. The crys- 

 tals cross one another at right angles when the twinning plane is the 

 prismatic one, and at 120° when the plane is octahedral. The crystals 

 vary in color from light to dark brown, and, though sometimes nearly 

 transparent, are often well-nigh opaque from the presence of impuri- 

 ties. Staurolite is abundant in the mica slates about Lisbon, and at 

 Mink pond in that neighborhood they are found loose in the soil, having 

 been washed out of the decomposing rock. Large brown crystals occur 

 at Franconia, and very large crystals at Charlestown. Mt. Washington, 

 Grantham, Bellows Falls, Walpole, Enfield, and West river, Vt., are 

 localities that are notable for staurolite, though they are found all along 

 the Connecticut valley. It is very often associated with garnet. 



Though the chemical composition of some varieties of staurolite cor- 

 responds to the formula given, it varies greatly, as is shown by the two 

 following analyses of staurolite from our state by Rammelsberg.* The 

 first is of a brown crystal from Franconia, and the second of one of the 

 clear deep brown crystals from Lisbon. 



Sp. Gr. 3.76 Sp. Gr. 3.41 



Franconia. Lisbon. 



Silica, 35-36 49.10 



Alumina, 48.67 yi -T^ 



Iron sesquioxide, 2.27 



Iron protoxide, . i3-05 10.69 



* Mineral Ckeinie, p. 590. 



