Wadsworth.] 232 [May 16, 



ELM ST., SOMERVILLE. WESTERLY " TRAP " DIKE. 



We have in this a finely porphyritic dike composed of augite, cli- 

 noclase, magnetite, and viridite, the viridite here being, as in all 

 other cases, a product of alteration. 



WIGGLESWORTH ST., OPPOSITE EVERETT AVENUE, SOMERVILLE. 



Several sections of this rock were made, showing it to be composed 

 of augite, clinoclase, magnetite, with viridite and a little olivine. 

 Signs of flow structure are to be seen under the microscope in some 

 sections of this and other dikes. In a section showing the junction of 

 the dike with the argillite, it passes from a compact gray base con- 

 taining crystals of feldspar (the crystals growing finer near the junc- 

 tion) to the argillite composed of rounded grains showing colors in 

 polarized light. The dike, at the junction, takes up little portions of 

 the slate and sends veinlets into it, the same as can be seen on a large 

 scale at the junction of the " diorite " of Maiden with the argillite. 



The greenish and grayish base or glass found at the junction with 

 the argillite and with the " greenstone " is undoubtedly altered tachy- 

 lite ; it is porphyritic from inclosed crystals of feldspar and augite, 

 and can be found on nearly all of the smaller dikes. 



EAST SIDE OF WALNUT ST., CORNER OF PEARL ST., SOMERVILLE. 



The dike here contains undecomposed olivine, which is readily 

 distinguished by the naked eye, the remaining constituents being the 

 same as those in the rock last described. 



ADAMS' LEDGE, SCHOOL ST., SOMERVILLE. 



The " trap " in this quarry is coarsely porphyritic from large twin 

 crystals of clinoclase, the composition plane being i-i, and each part 

 showing the characteristic striae. These crystals are usually rounded, 

 but show their contemporaneous origin with the matrix by containing 

 the same inclosed minerals that it does ; by being penetrated by, and 

 by inclosing, detached portions of the matrix; by crystals of apatite 

 and masses of magnetite extending through both the feldspar and 

 the matrix. The minerals inclosed by the feldspar, which is probably 

 labradorite, are apatite, magnetite, biotite, and graphite. 



The mass of the rock is like all the other " trap " dikes varying 

 from a gray to a dark gray, almost black mass, more or less compact, 

 breaking with a conchoidal fracture, and easily fusing to a black 

 magnetic mass. Microscopic examination of it in thin sections shows 



