MINERALOGY. 109 
part in the reaction, sphene is formed. The diorites and amphibolites 
about Littleton are rocks 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)? Al” Sif 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. 3.42 
Franconia. isbon. 
Silica, 7 ‘ 7 . ‘ ‘ é ‘ 35.36 49.10 
Alumina, . : ‘ : i 3 2 : 48.67 37-70 
Iron sesquioxide, . @ c ‘ - ‘ . 2:27 sichsne 
Iron protoxide, . : : . 2 c z 13.05 10.69 
* Mineral Chemie, p. 590. 
