212 TJie American Geologist. April, i898 
structure permits deep penetration of waters from the surface, 
so it affords easy passage to heated "minerahzers" from below, 
and the evidence from the pegmatytes in question shows dis- 
tinctly that they were formed by secondary infiltration. 
If this be true of the case in question, however, we should 
expect to find similar pegmatyte dikes cutting rocks in the 
vicinity other than the diabase. Such evidence is not want- 
ing; three miles to the westward the long ridge of Arlington 
heights, extending to the southward, is composed chiefly of a 
coarse hornblende-biotite-diortyte associated with an ancient 
amphibolite gneiss. Outcrops of the gneiss occur just w^est 
of the southern end of this ridge at Owl hill, and at numerous 
l)oints along its flanks, notably in the region immediately ad- 
jacent to Spy pond on the northwest Here the gneiss is in- 
terbedded with black calcareous strata and the whole series 
strikes northeast with a northwest dip of 40°. Intrusions of 
granite interrupt the gneissic rocks, appearing in one place 
like an irregular interbedded sheet or laccolyte; the gneiss is 
seen to dip directly under a granite mass. Cutting both the 
gneiss and the granite are pegmatyte dikes of coarse, granular 
quartz and large salmon-colored microcline crystals. Trans- 
verse to the plane of the wall, one of these dikes showed a 
remarkable parting, similar to columnar structure, in the 
quartz; the quartz could be broken out in rectangular blocks. 
As there is much evidence from thin sections that all of these 
rocks have been violently strained dynamically, it is probable 
that this jointage in the vein is due to pressure. These peg- 
matytes, of which several dikes were found, trend east and 
west as do also the basic dikes observed in the same vicinity. 
Minute veinlets of epidote occur in the pegmatyte, and epidote 
occurs in large idiomorphic crystals in both the granite of 
Arlington and the dioryte of Owl hill, conspicuously in the 
latter. 
These granites and diorytes are usually believed to be very 
ancient rocks, and so indeed they may be. The pegmatyte 
dikes cutting them, however, are identical in composition and 
microscopical structures with those cutting the Medford dia- 
base, and the two sets are believed by the writer to be of con- 
temporaneous origin. That this period followed closely on 
the intrusion of the granites is not probable in this case, and 
