wl 
181 
146 | N. 70? E. Vertical 1-1.25 | About 100/ S. of 144, in quarry ; 
porphyritic. 
N. 55° Е. N. 309 8 150' S. of 147; coarsely por- 
| phyritic and branching. 
146 | N: 70% W INN OT 1-1.5 Separated by short beach from 
145. 
147 | N. 759 E N. 5° 2-3 Near 146. 
148 5 
As a rule, the east-west dikes cut the shore ledges very 
obliquely ; and in going eastward we cross the first system 
from north to south, and the second system from south to 
north. The outcrops begin in the rear of the Black Rock 
House, where 88 is readily recognized by ite coarsely and 
distinctly porphyritic character. About 50 feet south of 88 is 
89, a splendid dike, which can be traced almost continuously 
for fully 750 feet. About midway of the outcrop it is some- 
what irregular and split up; and the well-preserved glacial 
striae upon it, and especially the freshness of an inscription bear- 
ing the date 1852, testify to the stable and resistant character 
of those diabase dikes which have, by deep-seated alteration, 
been largely changed to such almost indestructible species as 
epidote, chlorite, magnetite, and quartz. A little more than 
half a mile (2860 feet) from Forest Avenue brings us to 
90, the largest of all the east-west dikes; and its outcrop is 
quite a center of eruption (Fig. 17). It cuts one of the 
porphyrite dikes (81) and is eut by no less than four of the 
north-south diabase dikes. The small dikes (90% and 91) are 
probably branches of 90. The magnetic dike (94) is so 
highly charged with magnetite as to make the compass observa- 
tions quite unreliable. It not only cuts 85 and 86, but just 
before entering the sea is cut by 156 of the third system, which 
also cuts 95. “The next three dikes of this series (96, 97, 98) 
follow in regular order and bring us to Walnut Angle or the 
'astern end of Cohasset Rocks. No dikes have been observed 
in the ledges on Pleasant Beach; but near its eastern end we 
strike 100, which ean be traced the entire length of the head- 
land, trending just north of the extreme north end of Brush 
