204 Tlie Aincrican Geologist. April, i89.s 
The writer has recently studied in detail the h"agmental 
inclusions contained in many of the dikes of the Boston basin. 
Such inclusions of mineral substance, foreign to the matrix in 
which they are now^ embedded, frecjuently serve as excellent 
data for recording differential movement or process, affording 
a unit for comparison of the effects of secondary action which 
might otherwise be unrecorded in a homogeneous eruptive 
mass. The evidence, from inclusions, for the secondary or- 
igin of the micropegmatyte in the Medford "quartz-diabase" 
forms the subject of the present paper. 
The Medford diabase has been described by Wadsworth,* 
Crosby ,t Hobbst and Merrill.§ The rock extends as a broad 
dike from Somerville, northeast of Boston, Mass., through 
Medford, the next township to the north, for a distance of over 
three miles. In Medford it appears as a dike of width varying 
up to a maximum of several hundred feet; about the old Powder 
house, further south, in Somerville, the rock occurs again, 
immediately in the strike of the. Medford dike, but of unknown 
form or extent to the southeast; in this direction it is found 
again on Granite street in Somerville, not, however, in the 
Medford trend and again of unknown boundary. In Somer- 
ville the diabase is intrusive through the compact pelyte which 
forms the chief northern member of the Boston basin sedi- 
mentaries; to the north the dike cuts the older complex of 
granite, dioryte, quartzyte and felsitic flows and tuffs. 
■ Hobbs has described this rock as a diabase with an augite- 
dioryte facies; the latter occurs only in the outcrops about 
the Powder house and Willow avenue. Coarse ophitic ande- 
sine feldspars, with the triangular interspaces filled with aug- 
ite, biotite, large apatite crystals, the ores and a host of sec- 
ondary minerals, make up the normal diabase. The speci- 
mens of the dioryte facies from Willow avenue which we have 
studied differ from the description given by Hobbs in the 
large proportion of chloritized biotite and the absence of rec- 
ognizable augite. The hornblende is in large, deep brown. 
*Proc. Bost. Soc, vol. XIX, 1877, p. 217. 
fGeology of Eastern Mass. Occasional papers of Bost. Soc. Nat 
Hist., 1880. 
$Bull. Mus. Comp. Zool., vol. XVI, no. i, 1888. 
§Bull. Geol. Soc. Amer., vol. VII, p. 349. 
