Crosby. J 
502 
[May 18, 
many more or less distinct fragments of a similar or darker felsite 
and an occasional fragment of granite. The breccia-structure thus 
resulting is so marked in a portion of the rock that it was at first 
mistaken for conglomerate ; and the isolated elliptical area on this 
part of the map marked as conglomerate is really felsite. The 
red felsite occurs in an area which is covered almost continuously 
by salt marshes and drumlins ; and it presents only two obscure 
outcrops. There can be little doubt, however, that it forms a 
narrow east-west belt along the northern edge of the granite, 
extending east under Broad Cove and west beneath Squirrel Hill. 
Although there is no opportunity to study this interesting rock 
in situ , its effusive or volcanic nature is abundantly proved by 
its composition and structural features. It exhibits throughout 
a distinct but not conspicuous banding or flow-structure and en- 
closes many angular fragments of the same ora very similar felsite. 
Although the rock thus bears some resemblnnce to a breccia, it is 
essentiall} 7 identical in structure with some recent obsidians ; and 
it is undoubtedly an ancient, devitrified obsidian. Among the 
arguments against its sedimentary origin are the facts, that the 
fragments are all of the same kind of rock ; that they are never 
assorted or show in any way the action of water ; and that the fel- 
site is chemically intact, still retaining in every part the full pro- 
portion of alkali required for an acidic feldspar, which would be 
very unusual in a clastic rock. The fact that the fragments or 
so-called pebbles show a gradation in distinctness from those that 
are very sharply defined to those that are perfectly blended with 
the enclosing felsite, is only what should be expected when frag- 
ments of glass (obsidian) are enveloped in melted glass. 
GENERAL STRUCTURE OF NORTHERN HINGHAM. 
The eastern shore of Hingham Harbor is not only the natural 
boundary line between the geological districts of Nantasket and 
northern Hingham, but it probably marks the position of one 
of the great transverse faults of the South Shore ; and it certainly 
corresponds, as already explained, to a very important contrast 
in geological structure. The key to the structure of the vol- 
canic and sedimentary rocks of Hingham is the oblong area of 
granite and felsite lying north of the railroad and Beal Street. 
The general position of this mass i$ unquestionably anticlinal. 
