ART. 2. 



PETROLOGY AT GOOSE CREEK SHANNON. 



55 



These optical properties are practically identical with those given by 

 Larsen for celadonite" but the material contains no potash and 

 seems to be an ordinary chlorite. In places the material is oxidized 

 somewhat, giving a j^ellow-brown color, increased 

 birefringence, and higher refractive indices. En- 

 tangled with both this chlorite and the overly- 

 ing hornblende are abundant flat "hourglass" cry- 

 stals of epidote. 



Another shear zone from about 100 feet south 

 of where the last chlorite was found was exposed in 

 August, 1923. This shear, which intersected broken 

 and mashed but otherwise unaltered diabase peg- 

 matite, contained prelmite and areas of a fine scaly 

 soft gray-green chlorite, not immediately associated 

 with the prehnite. This chlorite resembles the stilp- 

 nomelane from Westfield, Mass., which I have 

 described,^^ but it is decomposed without oxidation 

 by boiling with 1 : 1 nitric acid. Under the micro- 

 scope it is seen to be made up of minute hexagonal 

 scales. Basal scales are dark in all positions be- 

 tween crossed nicols and in convergent light yield a 

 faint uniaxial or small biaxial negative figure with 

 the acute bisectrix normal to the plates. The indices 

 of refraction are oc = 1.625, j3 = 7=1.632. It is 

 pleochroic with X= clear bro\vn, Y = Z = deep blue- 

 green, absorption X < Y = Z. 



A single small portion of this material was ob- 

 tained in sufficient purity for analysis, yielding the 

 following results : 



Analysis of chlorite from shear vein. 



Fig. 5. — Albite; 

 showing prismatic 

 development o f 

 colorless trans- 

 parent albite cry- 

 stals occurring in 

 fractures and 



VEINS. 



"Esper S. Larsen jr., Microscopic Determination of the Nonopaque minerals. U. S. Geol. Survey 

 Bull. 679, p. 257, 1921. 



23 Earl v. Shannon. Diabantite, stilpnomelane, and chalcodite of the trap quarries of Westfield, Mass. 

 Proc. U. S. Nat. Museum, vol. 57, pp. 397-403. 



