Wadsworth.] 228 [May 16 



building stone. It is the principal building stone at Medford, and it 

 can be seen on many of the streets in Boston, particularly on the 

 older and cross streets. My thanks are due to President Eliot for 

 information upon this subject. 



Thin sections of the rock examined under the microscope show 

 that it is composed of a triclinic feldspar, augite and magnetite, with 

 apatite, biotite, viridite, and pyrite. The feldspar, augite, and mag- 

 netite are the essential constituents, while the viridite, and probably 

 most of the biotite if not all of it, are the products of alteration. 



The feldspar is much decomposed, being altered to viridite, preh- 

 nite, and kaolin, but shows in polarized light that it is clinoclase; the 

 alteration to viridite commences usually in the interior of the crystal, 

 and may be, as well as prehnite, which is of similar occurrence, the 

 occasion of the appearance of much of the feldspar, which has a 

 pink tinge on the outer part, and a greenish one in the centre, al- 

 though this is generally considered to be owing to two different 

 feldspars. 



The augite occurs in columnar crystals penetrating, and interpen- 

 etrated by, the clinoclase, showing the contemporaneous crystalliza- 

 tion of both. It is divided by an irregular network of fissures, and 

 from these and the surface of the crystal the alteration to viridite 

 begins, and crystals are to be seen in every stage of alteration, from 

 those that are partially, to those that are wholly, changed. 



The viridite arises from the alteration of the feldspar and augite. 

 Its first appearance is amorphous, it then assumes a fibrous structure, 

 and finally passes into biotite and other definite forms. The weight 

 of the evidence would go to show that the viridite is the intermediate 

 state that the minerals of this rock pass through in the course of 

 their alteration to definite compounds. It is often found pseudo- 

 morphous after the augite. 



The biotite, although it may have been partly an original constit- 

 uent of the rock, comes from the alteration of the augite either 

 directly or else passing through the viridite stage, and the evidence 

 goes to show that the latter is the case. The reasons for assuming 

 this are these: The apparent gradual passage from augite through 

 viridite to biotite, as seen under the microscope; the general non- 

 appearance of biotite only in those parts where the augite has been 

 greatly or entirely altered; [its comparative absence in the least 

 altered portions of the rock; the abundance of the viridite in the 

 partially changed parts, and finally the almost entire absence of the 



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