358 PROCEEDINGS: BOSTON SOCIETY NATURAL HISTORY. 
another very rapidly, though the contacts of intersection are well 
marked. Three series, and a possible fourth, were intruded before 
the Medford diabase dike, and one small dike was intruded sub¬ 
sequent to it. In mapping the dikes older than the Medford diabase, 
no attempt has been made to indicate, by means of a color scheme, 
their relative ages. Where this has been positively determined in 
a few localities it has been indicated by showing the intersections of 
■ the dikes concerned. In other localities than those where the rela¬ 
tionship is indicated by truncation, it will be necessary to correlate 
the dikes on lithological grounds. Sometimes this distinction may 
be made macroscopically, but in most cases it is necessary to refer 
the specimen to the microscope. 
The two older members of this series are phanerocrystalline in 
texture, while the two younger may be described as aphanitic. All 
but the youngest carry visible amounts of pyrrhotite, the youngest 
of the three having the least. The oldest dikes contain a pink 
augite, of unusually low index of refraction, in small aggregations, 
and a brown hornblende; in some cases there is a parallel growth 
of the two silicates filling the interspaces between idiomorphic 
})lagioclase crystals. There is also an abundance of chlorite, second¬ 
ary from augite and hornblende, and a considerable quantity of 
ilmenite with titanite as an alteration product. The iiext youngest 
member of the series is tinted a greenish-gray color by the presence, 
in considerable amount, of a pale green augite (almost colorless in 
thin section), probably diopside; hornblende seems to be entirely 
absent, while chlorite, epidote, and a considerable amount of calcite 
.are present as secondary products. The third member of the series 
is fine-grained and black, and exhibits, in thin section, an intersertal 
structure, whereas that of the two older is ophitic. The augite is 
violet in color; magnetite occurs in considerable quantity, also 
calcite, chlorite, and actinolite, the last-named in the form of 
secondary needles mingled with the chlorite. 
In several localities there are dikes which cut some one or more 
of the three preceding members of this series. They are dense, fine¬ 
grained, black in color, apparently free from sulphides, and exhibit 
a hyalopilitic structure with rods and small masses of augite in a 
glassy matrix. These are olivine-free basaltic dikes. Magnetite 
is present in considerable amounts. Some of the dikes carry a con¬ 
siderable quantity of brown biotite, together with the augite, plagi- 
oclase, magnetite, and secondary and accessory minerals. 
