127 
but а more extended study would be required to discover its 
full significance. An occasional dike only, such as 92, 109, 
114, 136, ete., is distinetly aberrant or ambiguous in trend ; 
and some of these afford intersections determining the relative 
age and hence the system. Although faults are, perhaps, ав 
numerous in Cohasset as in Nantasket, none have been 
definitely located; and we can only conjecture that their 
relations to the dikes are unchanged. The dikes are often 
observed to coincide with prominent joint-planes in the granite ; 
but as at Nantasket, the joint-structure is evidently mainly of 
more recent origin than the dikes. 
The Porphyrite Dikes. 
Width 
No. | Trend. Hade. in Remarks, 
| feet. 
| 
71 | N; 6229, E. 159-209 | 4 In granite on Green Till Beach. 
72 | N. 809 E. Ij, 109 8 750 E. of 71; irregular. 
TE NS ADS Е. Vertical 18 15“ E. of 72; imperfectly ex- 
| posed. 
74 | N. 89 F. Vertical 2 250! E. of 73; near Black Rock 
| House. 
75 | N. 18" Шу E. 8° 8 Near Black Rock House; cut 
| һу 88. 
10 N. p> I Vertical 13—14 Behind Black Rock House; 
cut by 88 and 89. 
T ING 40° D. N.W.10°-159 4—5 Branch of 76; cut by 88 and 89. 
78 N.-S. Vertical 1-6 About 450! E. of 77; cut by 89. 
79 | N.-S. Vertical 10—15 
80 | N. 20? E. Vertical 8--10 About 2550! E. of Forest Ave. ; 
cut by 150. 
81 | N.-S. Vertical 8-10 About 8000! Е. of Forest Ave. ; 
cut by 90 and 91. 
82 N. 20° Е. | Vertical 1-1.5 About 450! R. of 81. 
88 | N. RO" E Vertical 2-8 6' east of 82; probably con- 
nected with 81. 
84 | N. 15° E. Vertical 6 Near 88; cut by 92 and 93. 
8 N. lO K. Vertical 25-30 About 100! E. of 84: cut by 94. 
SO UNS LOS B Vertical 3-4 About 200! E. of 85; also cut 
by 94. 
Sii UN VADO. ç Ми ДО” 8-8.5 West end of Pleasant Beach; 
cut by 97 and 98. 
The dikes of this series have not been observed along the 
entire shore, but only for about one mile, between Green Hill 
Beach and Pleasant Beach or Walnut Angle. This is the 
comparatively straight and eminently rock-bound part of the 
