1877.] 231 [Wadsworth. 



The augite is much decomposed, the viridite is very abundant, 

 while but little biotite shows in the thin section. The apatite is 

 partly altered to viridite. Near the contact with the argillite the 

 rock is compact and fine grained, looking like some of the coarser 

 " traps," but a microscopic examination of it shows that it is the same 

 as the coarser parts, except that its constituents are less decomposed 

 and it contains a little quartz. 



" Trap " Dikes. 



BRIGHTON, NEAR THE WATER WORKS. 



Of the four dikes here, one is about eight or ten feet in width, and 

 at one end shows the same concentric disintegration that the " dio- 

 rites " have, while the other end shows the usual prismatic jointing 

 common to such dikes, the columns being perpendicular to its walls. 

 This dike is a vertical one, but many of the dikes are inclined 

 towards the east. Thin sections under the microscope show that this 

 is of the same composition as the " diorite," particularly the compact 

 part, near the argillite at the " Powder House," Somerville. It con- 

 tains considerable viridite. Its structure and composition show that 

 it is the connecting link between the " diorite " and " trap." Some 

 prehnite is found in this dike. 



WALNUT ST. BETWEEN BONAIR ST. AND BROADWAT, SOMERVILLE. 



Thin sections show that the third dike from Walnut St. is com- 

 posed of a reddish brown augite, clinoclase, magnetite, olivine and 

 viridite; the viridite forming pseudomorphs after olivine, or partially 

 replacing it. This dike and the middle one in this quarry, are finely 

 porphyritic from elongated crystals of feldspar, while the other trap 

 dike in this quarry is mainly uniform in texture. The middle dike 

 contains clinoclase, magnetite, and viridite, forming a matrix in 

 which larger twin crystals of clinoclase are enclosed, also partially 

 altered crystals of olivine. But little unchanged augite is to be seen. 



BOND ST., SOMERVILLE. 



The " trap " here is composed of augite, clinoclase, magnetite, viri- 

 dite, and also olivine, partially or wholly altered to viridite. In this 

 there is a tendency to the formation of little spheres of decompo- 

 sition, tending to give the rock a variolitic appearance. 



This same structure is very prominent in some of the Roxbury 

 dikes on Tremont St., and in Somerville, on Elm St. 



