17 | 
Scituate shore, near the Osher Rocks, and another on the Co- 
hasset shore, north of Sandy Cove; but inland nothing deserv- | 
b ing the name of a quarry has been observed. The third granite, 
| in chronological sequence, forms small and irregular dikes 
| from a fraction of an inch to several feet in width cutting all the 
| older rocks, but occurring chiefly in the coarse, typical granite 
| (No. 2), from which it appears to differ in composition only in 
containing less hornblende or mica. These dikes are always 
fine-grained, varying from a finely crystalline gray or pinkish 
granite or micro-granite to a true felsite. 
The miero-granite passes into an acid felsite of the same chem- 
ical composition. But while the micro-granite is common, form- 
ing hundreds of small dikes, true felsite, which does not reveal | 
a holocrystalline ground-mass under the microscope, is rather I! 
rare. The largest mass which has been observed is at the || 
northeast corner of the large ledge of granite on the south 
shore of Lyford’s Liking, south of Round Hill. This is a 
brown, thoroughly compact or felsitic, structureless rock: and 
. — 
forms a mass several yards across, just at the water’s edge. 
The contacts with the granite are not clearly exposed; but it is 
evidently a dike in the granite. Small dikes of a true quartz- 
porphyry have also been observed on the Jerusalem Road, east 
of Green Hill. The great abundance of pebbles of felsite in 
the Nantasket conglomerates indicates that this rock was for- | 
merly much more extensively developed in this district. It | 
probably occurred chiefly in the form of broad surface flows, | 
Such as still exist їп other parts of the Boston Basin; and 
the dikes of both felsite and miero-granite probably date from | 
these volcanic eruptions, being branches from the main fissures 
or necks through which the felsite reached the surface. | 
Although these three types of granite clearly reveal the | 
chronological succession described above, it can not be shown | 
that they are widely separated in time or belong to entirely 
distinct periods of igneous activity ; and it is especially obvious 
| 
| 
` .` . | 
that the micro-granite and felsite, although cutting the coarse | 
` OCCAS. PAPERS B. S. N. H. IV. 2 
