B. K. Emerson — The Deerfield Dyke and its Minerals. 353 



itself, which is there quite fresh, and where slickensides occur 

 they are of light gray color, due to the mixture of datolite and 

 trnp, without the appearance of any of the chloritic or serpen- 

 tinous products of decomposition which darken the same for- 

 mation in the prehnite. More commonly it rests upon a thin 

 layer of massive prehnite, rarely with several alternations of 

 and prehnite in calcite or upon the thick layers of 

 shaped crystals of the former mineral, where the sepa- 

 ! faction'. The crys- 

 tals of datolite, which resemble those of Farmington in their 

 dassv clearness and general habit, are remarkable for the great 

 number of faces represented and for the distinctness of the 

 types into which they are grouped, as well as for the manifold 

 and intricate forms produced by the unique development 

 sometimes of single faces and sometimes of all the faces on 

 one side of the vertical plane. 



The following is a list of all the forms represented, arranged 

 according to the table given bv E. S. Dana * n (a), ti {b\ O (c), 

 I'.l/..<-2 ..0. „;j(/,. -I H,/).-4-/(o.-2-/lrK-«i-/(*),4. l -(/7), 

 2-Mc), $\a>) !->(*,. £-■,(/,. L>M.v) 4-/( W ) -4 1 . 2u) ju), 

 1 i//, i <>,. _(j.3 uy , _4-2 u,\ 4 2 ill). 12 J (-), -l.i-2 (R), -8-2 

 .'■■ *-- «>>. | 2(h), -2-2 <p, 2 2 (,/.,. «-*/(/), 8-2(^,f2(0.) 

 L'^.HU-:.. [::Mm. 3-mK). i'-'.'.(!;. 



i'"i' convenience of description the crystals may be divided 

 into three types: 



I. 77/e ri'fjitbtr. — fn this almost spherical forms are produced 

 b J the equal development of a large number of faces. In 

 one crystal x, e, m were slight! v larger than the rest ; a, M, n, 

 °i A Q, equally developed c, q, B, very small. 



Another very stout, prismatic and resembling the Andreas- 

 berg crystals somewhat in the distribution of its faces, except 

 that the elongation in the direction of the clinodiagonal axis 

 was only slight, had the faces >■, s, /. </. m, largest; //. M, -4, # 

 '^■/\ ; <- about equal .^ «, i- 2 , 0, £ < 



•Tthol 



nate. 



A fourth minute but very beautiful crystal was unique for 



l,,is loenhtv in having the fare „ large and square and ii elon- 

 gated vertically, the laces fj. m, n, x, a, n, M, e, X, o, p, Q, U, a 

 "":'" «'«i'i.-dh- large, f ?, ^, /, F small. 



Several of the larger ervstals of this tvpe showed a new 

 f^ i» the /one / <|), ,, (2-h and o (i2). c\0) = f-2 to which 

 1 nave in the precedmg list applied the letter C. 



Il; ;vc in the preceding lii 

 h- Prismatic type. — By 



J i- / nsnuttic iijj"'.- -Bv the great enlargement ot x and tw 

 opposite faces of w, a prism of 116° is formed, elongated i 



A«. Jour. Sci.-Third Series, Vol. XXIV. No . I^.-Xovembek. 188 . 



