of Boston and its VicinitiJ, 203 



r 



and vitreous lustre. The colour of the basis is greyisli black ; it 

 is hard, difficultly frangible, and the fragments are indeterminate- 

 ly angular and sharp edged. Although it resists decomposition 

 more than common Greenstone, it is paitially affected by atmos- 

 pherick exposure, and becomes covered with a brownish crust. 

 This is the superb black Porphyry of the ancients, and occurs in 

 small beds in Argillite, and in rolled masses, at Charlestown j 

 and in veins in Greenstone, at Marblehead. 



II. Green Porplujnj. 



Cleaveland, p. 6ll. Jameson, vol. iii. p. 13]. 



The basis of this beautiful mineral has a uniform simple ap- 

 pearance, and requires the aid of glasses to discover its compo- 

 nent ingredients. Its colour is of a uniform leek green, or black- 

 ish grey ; it contains imbedded crystalline grains of Feldspar, 

 which sometimes intersect each other, and sometimes two or more, 

 proceed from a centre like radii. Some specimens resemble Ser- 

 pentine and Petrosilex, and are analogous to the antique green 

 Porphyry, so highly valued by the antiquary. It occurs in Dor- 

 chester, Brookline, and Roxbury, in rounded masses, and in 



small quantity in veins at Marblehead, associated with the first 

 variety. 



XI. Greenstone in this vicinity has not been observed strati- 

 fied. It forms gently rising hills, which sometimes present high 



F 



mural precipices. It overlays Argillite, and some of its varie- 

 ties occur in large beds in it, at Charlestown. It passes into Si- 



* 



enite at Weston, VValtham, Stoneham, and Dedham ; and it is 

 frequently interrupted by transverse parallel rents which inter- 



L 



sect each other obliquely 5 hence rhomboidal masses are formed. 



