1877.] 233 [Wadsworth. 



that it is much decomposed, and has a flow structure. It is composed 

 of clinoclase, magnetite, a very little unaltered augite, olivine, viri- 

 dite, apatite, and pyrite, pyrite occurring in all the intrusive rocks 

 described in this paper. Porphyritically inclosed in the matrix, be- 

 sides the feldspar, occur biotite, graphite, olivine, pyrite, hematite, 

 apatite, serpentine, and magnetite. The graphite is in fine pure 

 scales, the hematite arises in some cases from the decomposition of 

 the olivine, which is here of a brownish yellow color, and on heating 

 turns black and becomes magnetic. The apatite occurs of a brown 

 color and in hexagonal crystals, both tabular and columnar, also in 

 rounded columns and grains. The magnetite on test showed no trace 

 of titanium. The olivine and magnetite contain inclosed crystals of 

 apatite. Calcite and other products of infiltration and alteration are 

 to be found. 



" Greenstones." 



MALDEN, NEAR UPHAM ST., CORNER OF FERRY. 



Thin sections show that this rock is composed of augite, feldspar, 

 magnetite, and viridite. The feldspar is so altered that no trace of 

 its triclinic character is visible, and in other sections of this the con- 

 stituent minerals are much more decomposed and changed to viridite. 



MEDFORD, CORNER OF DEXTER AND MAIN STS. 



This "greenstone" contains much augite, feldspar, and magnetite 

 with a little quartz and calcite. 



SOMERVILLE, WILLOW ST. 



The feldspar here, although much decomposed, still shows traces 

 in polarized light of its triclinic character. Hornblende of a yellow- 

 ish color in the thin sections is found, and appears to be an alteration 

 product. The rock further contains apatite, viridite, biotite, and 

 magnetite, but hardly a trace of augite. 



SOMERVILLE, CORNER OF LOWELL ST. AND SOMERVILLE AVENUE. 



The general structure of this and of part of the other "greenstones," 

 is essentially the same as that of the " diorites," and it is composed 

 of feldspar, viridite, magnetite, apatite, augite, and traces of horn- 

 blende. The rock near its contact with the slate is of a dirty gray 

 color, and under the microscope shows a gray base with crystals of 

 feldspar porphyritically inclosed. 



