128 
shore known as Cohasset Rocks. Although it is convenient. to 
class all the dikes of the above list, provisionally, as porphy 
rite, it is well to bear in mind that a few at least are more 
basie and that the list quite possibly includes some melaphyr 
dikes. "The chief points of interest. presented by the porphy- 
rite dikes have been noted in connection with the Black Rock 
flow ; while most of the details of any importance, including 
especially those required for the identification of the individual 
dikes, are presented in the table. We learn from the table 
that nine of the seventeen dikes are visibly cut by the oldest 
system of diabase dikes. On account of the approximate 
agreement in trend, intersections by the north-south diabase 
dikes were scarcely to be expected ; but, fortunately, one such 
is presented ; and it is, perhaps, the prettiest and most unique 
phenomenon to be observed among all the dikes of this district. 
The relations of the two dikes are shown in Fig. 16. The 
diabase dike (150) advances obliquely from the south until it 
strikes the east wall of the porphyrite dike (80), follows this 
wall for 25 feet, amputating a branch of the porphyrite dike, 
and then passes in a graceful double curve diagonally through 
the latter and follows the west wall as far as either can be 
traced, —24 feet. The intersecting dike is a typical example of 
the third system of diabase dikes—black, brownish-weathering, 
and beautifully eross-jointed. This porphyrite dike (80) is the 
one affording the analysis given on page 59. On the north 
LEELA TES AAKE O k EA E E A уке, 
X X » XXX is Wi x n 
x y y X 00. E: ah 
e 
Fic. 16.— PLAN SHOWING THE INTERSECTION OF A PORPHY- 
RITE DIKE (80) BY A DrABASH DIKE (150) 
ON THE COHASSET Rocks. SCALE, 
L INCH = 25, FEET, 
