of Boston and its Vicimtjj, 



«) 



01 



-^ 



IV. The grains of Feldspar and Hornblende vary in size from 

 very small to coarse ; hence the structure of Greenstone presents 



b ■ 



a great variety, from coarse to fine grained, and from crystalline 

 to earthy, in which the two ingredients are with difficulty distin- 

 guished. Generally, the grains are middle sized and easily per- 



ceptible. 



V. The colour of the fine grained varieties is 

 black, or dark leek green ; these break into angular fragments, 

 with sharp edges ; and have sometimes a slaty structure, which 

 arises from the great proportion of Hornblende ; and where this 

 predominates, the variety has a greyish black colour, with an as- 

 pect and structure approaching to Gneiss. 



VI. Common Sulphuret of Iron occurs in small quantiiies, in 



either greyish 



cubic 



miated in Greenstone 



cales of black 



Mica, Quartz, Prehnite, Epidote, and Calc Spar occur in it, 

 either massive, disseminated or in veins. 



VII. Greenstone is liable to decomposition ; that which contains 

 the Mica is the most, and the firm, compact, dark leek green vari- 

 eties, are the least subject to this change. At Powder-house hill, 

 in Charlestown, in the centre of Medford, in Readin 



g, and in 



Vi 



Green 



forms a beautiful 



pletely disintegrated 

 which is much emp 



yed 



for forming hard gravel walks. At these places, the Greenstone 

 occurs in large globular masses, with a solid nucleus, -surrounded 



by concentrick lamina of Gr 



ges of 



position; the lamina are of various thickness, and are often ea 

 ly separated. It bears some resemblance to that variety of sec 

 dary Greenstone, called Gi ^bular Rock. Globular masses app 



piled on 



each other, like 



all, and 



are 



filled with the above mentioned reddish sand 



