168 BULLET^' OF THE 



amount of the ground mass 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 (noi'theast), 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 debris, 

 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 debris 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, "^nd the action of the frost and the 

 ocean waves, is much decomposed ^a 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. 



