93 
[Lane. 
of which may perhaps be from the gap between Little and Great 
Nahant. On the northwest side of Little Nahant is another out- 
crop of sediments — seams of quartzite, — in some of which clastic 
grains of sand may yet be seen, and green slates of a waxy lustre 
like semifused specimens from the Oetzberg in Hesse. 
Upward of five hundred dikes have been noted. They form 
probably from three to six per cent of the rock. They are mostly 
dike forms of diabase. The exceptions are, first, a few small dikes 
of diorite porphyrite (one such occurs on the east side of Bailey’s 
Hill), which contains much reddish orthoclase or microcline in the 
ground mass. Secondly, there is a dike running about east across 
John’s Peril, which is light green and dense. It weathers white, 
when the otherwise scarcely noticeable porphyritic crystals of feld- 
spar make pits. It contains zirkon and gives a very strong sodium 
flame. It is a felsyte or more precisely, perhaps, a keratophyre. 
The diabase dikes differ widely in looks, but all have a fine or 
coarse lattice of lath-shaped feldspar. Two or three hundred feet 
east of the steamboat wharf, there is a dike in which the laths 
are at times an inch long and very glassy. The southeast corner of 
Bailey’s Hill is formed by a very large dike of what has been called 
labradorite porphyrite. The French name plagiophyre is simpler 
and more inclusive. The metalloids here and in many not ultra 
basic dikes occur in tree-like forms. 
Just southwest of Pulpit Rock, striking northwest, is a good ex- 
ample of another class of dikes which have some of the bisilicates 
in porphyritic crystals. Besides augite, this particular dike has 
cavities where was once olivine. They are now filled with amphi- 
bole needles. I suggest the extension of the name lamprophyre 
for these dikes. 
A transition to the lamprophyres, according to Rosenbusch’s 
original definition, is made by a dike at Black Mine, which has a 
strike N. 12° E., in which case porphyritic crystals of diaclasite and 
olivine pseudomorphs lie in a base containing much ruddy mica 
and olive brown hornblende. This dike stands near the kersantites, 
but I suspect it is affected by a solution of inclusions. 
The dikes are altered not only by atmospheric weathering, but 
also dynamically, a schistose structure being produced with well 
known mineral metamorphoses. This effept may be especially well 
studied at Bailey’s Hill. 
Robinson’s history of Lynn gives the following list of minerals 
