Minerals in Greenstone. 



433 



the greenstone is separated from the limestone in some places only by a 

 narrow strip of sandstone : And a part of the limestone is more or less 

 frequently converted into tripoli : that is to say, the carbonic acid is 

 expelled, leaving the argillaceous and silicious matter by itself. Prob- 

 ably this was the effect of heat : though I am not very confident that 

 the tripoli was produced in this manner. A part of the limestone at 

 that place is very much indurated, so as to possess almost the brittle- 

 ness of glass when broken. 



Mineral Contents. 



It is not unexpected, though gratifying, to find in our greenstone 

 the same minerals as occur in the trap rocks of Europe. 



In describing the new red sandstone I have given an account of sev- 

 eral veins in that rock of sulphate of baryta and native copper,green car- 

 bonate of copper, with pyritous copper and the red oxide of that metal. 

 These veins often extend into the greenstone a considerable distance. 

 But for the particular localities I would refer to the description already 

 mentioned. 



According to the Messrs Danas,* a vein of magnetic oxide of iron 

 occurs in the greenstone at Woburn ; though not extensive enough 

 to render it an object for the miner. Intimately mixed with this ore 

 is pyritous copper : and this last is invested sometimes with the muri- 

 ate of copper. Quartz and amygdaloid at Brighton, and rolled pieces 

 of granite at Medford, contain the same mineral. They state also 

 that micaceous oxide of iron exists in the greenstone at Charlestown. 



According to the same gentlemen, asbestus is found in fragments of 

 greenstone in Brighton and Dedham ; and I have found it in one of the 

 anomalous varieties of this rock at Nahant. Probably however, it is 

 comparatively rare. 



Epidote, as already mentioned, exists abundantly in the greenstone 

 around Boston ; but never to my knowledge in that in the Connecti- 

 cut valley. Generally it is disseminated through the greenstone ; but 

 sometimes it occurs in veins, and is then usually compact, though of- 

 ten crystalized. At Breed's Hill is a locality ; and a much better 

 one at Nahant. 



The cavities of the amygdaloid are sometimes occupied by a dull 

 green foliated mineral which appears to be chlorite. The folia have 



Mineralogy and Geology of Boston, &c. p. 66. 



' 55 



