354 B. K. Emerson—The Deerfield Dyke and its Minerals. 



the direction of the axis of the zone x, o, m, the sharp edges 

 of which are beveled by n and o elongated into narrow planes. 

 The other planes are grouped in a very confusing way around 

 the ends of this prism. The uniform dullness of the face x is a 

 characteristic feature. 



The crystal under this type which was richest in faces 

 showed the following combination a, b, c, M, o, x, g, m, w, e, X, 

 ft, q, /9, Q, U, B, a, i; another lacked the faces b, k, and those in 

 the zone B, o, /?. Other crystals showed the faces E, jt, and r, 



III. The tabular type. — The roughened faces of x are large 

 and nearly circular and approached so as to reduce the crystal 

 to a thick plate, around the edge of which the other luces are 

 placed. The crystals of this type resemble closely the Hay- 

 tori te pseudomorphs, some of them having the same faces in 

 the same relative development as in the latter, others being 

 even richer in forms than any I have seen described from the 



One costal contained twenty-eight distinct forms, several 

 only as fine lines, but all capable of quite close measurement. 

 The crystal was peculiar in having the face a (2-2) striated in 

 two directions, forming a series of V's, as is common with the 

 face o from other localities. The latter face is here finely 

 polished. This striation was apparently parallel to the inter- 

 section edges which the face a produced would make with o 

 and i-i. The forms present were a, c, M, o, u, s r , II, £, Q, <x, 

 g, m, n, s, X, ft, x, g, 6, ft Q, U, a, B, E, F, K, G. Another 

 crystal of this type went to the extreme of simplicity, being 

 bounded by the faces a, c, M, o, f, g, m, n, e, X, ft, q, all quite 

 large except q and 0, and the face £ unusually nir-v and shield- 

 Similar tabular forms have been recently described from the 

 spheres of chalcedony from Theiss in Tyrol.* 



Enclostjkes ix Datolite. Calcite. — When the datolite is 

 thrown in acid much calcite is dissolved and a vesicular mass 

 left ; and similar pieces are found in the vein itself, showing that 

 the same operation has been performed by natural agencies. 



Sfdniite? Baritef — In the thick veins the minerals are often 

 abundantly gashed by the removal of broad, thin blades of some 

 mineral, possibly selenite or barite, and the surfaces drused 

 over by minute, very distorted crystals of datolite. 



Axinite. — Some of the finest crystals of axinite occur in the 

 datolite. The crystals are here sometimes short stout prisms. 

 Prehnite. — Small portions of prehnite are enclosed rarely in 

 the lower portion of the datolite, also small patches of the 

 chloritic mineral identical with that derived from the decom- 

 position of the former. 



* C. Vrba, Zeitsch. Kryst., v, 425. 



