168 BULLETIN OF THE 
amount of the groundmass and a few full fluid cavities. It is a simple 
crystal of sanidin.” 
Dr. Wadsworth’s field-work upon this formation was very thorough, 
and but little is required in addition to his clear description of it. A 
few notes, however, taken from his description and the observations of 
the writer, may be of interest. This formation, now determined to be 
keratophyre, can be seen at low tide near the residence of Mrs. Harding 
on Boden’s Point, Marblehead Neck. It appears as the much eroded 
remains of a surface flow, and extends two hundred yards in a north- 
easterly direction, with a width of sixty feet at the lowest point of ob- 
servation. There are smaller masses of this rock three hundred yards 
from this point in the same strike (northeast), which are exposed only 
at extremely low tides. About five hundred yards south of Boden’s 
Point, near Flying Point, the eruptive granite cuts the metamorphic 
slate of the Boston Basin series, and near this point also the granite is 
cut by dikes of quartz-porphyry (felsite). Near the keratophyre, and 
dipping under it, is a banded felsite. Both the granite and the felsite 
are cut by diabase dikes. The felsite tends to the northeast, and forms 
the larger portion of the bed rock of the Neck. The banding of this fel-. 
site dips towards the harbor nearly north, and lying upon it is the kerato- 
phyre. Between the lowest points of observation and the banded felsite, 
a conglomerate of varying thickness composed of fine felsitic débris, 
holding rounded and angular fragments of the felsite, is found in several 
places enclosed in the keratophyre. In some places the keratophyre 
rests directly upon the felsite, while in others the conglomerate inter- 
venes between them. The line of contact between the keratophyre 
and the felsite débris is well marked; specimens of the keratophyre 
detached at this point show a basal surface very rough and pitted where 
it conforms to the irregularities of the conglomerate. The keratophyre, 
being exposed to the sun, rain, and the action of the frost and the 
ocean waves, is much decomposed on the surface ; but the least altered 
specimens obtained are of a brownish or bluish gray color, having a 
conchoidal fracture and a compact groundmass, holding, occasionally, 
large glassy crystals of anorthoclase, some of which are one fourth of an 
inch in length, and, rarely, plates of biotite. The groundmass in thin 
section under the microscope is shown to be filled with lath-shaped feld- 
spar crystals, which are somewhat decomposed. The base is an earthy 
kaolinized mass, with irregular masses of quartz and earthy limonite. 
Dr. Wadsworth described the rock from microscopical study as con- 
sisting of a groundmass composed of ledge-formed crystals of feldspar, 
