Wadsworth.] 230 [May 16, 



feldspar and prehnite, and sometimes seems to be formed without 

 passing through the viridite or green bronze stage. On the feldspar 

 and prehnite it occurs sometimes as stellate coatings on the unaltered 

 mineral, at others in stellate groups in little pits upon the surface, as 

 well as replacing the minerals in part, or as a whole. In one feld- 

 spar crystal the alteration was observed to take place in layers par- 

 allel to O, (001). 



The remaining minerals are principally magnetite and pyrite; the 

 magnetite, when tested, showed no trace of titanium. The melano- 

 lite of Wurtz is deposited on the sides of fissures by the infiltrating 

 waters, and assumes a striated, imperfectly fibrous shape, and gives 

 rise to faces and appearances, similar, if not identical, with much of 

 the so-called " slicken-sides"; on long exposure it changes from its 

 original to a rusty brown color. 



An examination of many of the rocks in this vicinity would indi- 

 cate that a large number of the faces that are here called " slicken- 

 sides " arise from a deposit of epidotic, chloritic, or other mineral 

 matter deposited by water on the surface of pre-existing fissures, and 

 which may or may not have been further smoothed by the attrition 

 of running water, or by the expansive force of the water freezing in 

 the fissures. 



MALDEN. 



The Maiden locality is not now worked ; the rock has been used to 

 form the wall on the north side of the " Old Cemetery," near the 

 quarry. The contact with the argillite can be seen here. The argil- 

 lite is much indurated and resembles felsite. Thin sections show the 

 rock to be composed of clinoclase, augite, magnetite, viridite, with 

 some biotite, pyrite, and apatite. The clinoclase is very much al- 

 tered. This rock, in all of its essential characters, is the same as 

 that last described. 



MEDFORD. FOREST STREET. 



The specimens from this locality are mainly surface ones, therefore 

 much altered, yet thin sections show its composition to be the same 

 as the preceding. 



The augite has in a great measure disappeared, and little viridite 

 is found, but the biotite is very abundant. 



SOMERVILLE. POWDER HOUSE. 



The general structure and composition of this rock is the same as 

 that of the foregoing localities. 



